The debut of Stephen Strasburg is one that baseball fans have been waiting to see. Assuming you're reading this, you'll be watching as well (MLB Network, check your local listings.) So now that we've heard all the hype and seen glimpses of him in scouting video and in the minors, what should we be watching for when he actually takes the mound? Get your popcorn, scorecard, and ready, because here's your viewers guide:

1. Fastball
Everyone comes to see the fastball. It's why he's famous, and with good reason. Strasburg's fastball has been credibly recorded as high as 102 and lives in the mid-90's. The fastball comes with all five elements: velocity, control, command, movement, and touch. Velocity is the easy one, so let's define the other three. Control is the ability to hit a spot or, more specifically, to throw strikes. Command is more loosely defined, but most agree it's the ability to move pitches around within a controlled area. Leo Mazzone preached command to his Braves pitchers, putting balls in specific spots in and out of the strike zone in order to make hitters swing at their pitch, so that even if contact was made, it was weak. Greg Maddux famously threw pitches in order to set up players for later at-bats, while Tom Glavine would work as much on the umpire as he did the hitter, drawing the strike zone further and further out. 

Movement has long been the tougher one to define. Hitters left muttering as they walked back to the dugout knew that the ball had tailed, faded, or cut, but there were few outside that small number, and an equally small number of scouts, who could credibly judge movement. With the advent of Pitch-f/x, there's now a way to measure and quantify this movement, allowing comparisons and drawing out for us what hitters have dealt with for years. With Strasburg and a few others who touch velocities most of us can only dream of, there's an additional combination element. Many think that faster pitches have less movement, something noted in my report earlier this year on Aroldis Chapman. I asked Pitch-f/x guru Harry Pavlidis if this was the case. The problem, he told me, is in the sample size. There are very few pitchers with this kind of velocity, and fewer still that also feature a breaking ball to go with it. Harry focused on Justin Verlander as the best comparable, but found a very weak correlation between horizontal movement and fastball velocity. Many will be watching the Pitch-f/x stats for Strasburg's first start to see just how much his pitches move.

Touch is the final element, and one that is perhaps more apparent in Strasburg than most pitchers. Because of the variance between his "normal" fastball and his "max" fastball, Strasburg effectively turns it into multiple pitches. The touch on his pitch, his ability not only to control the placement and location of the pitch, but to change the movement and speed at will, disrupts the timing of the hitter. "Part of it is mental," an advance scout for an NL team told me. "A guy that's been told it's coming at 100 is cranked up for 100. He gets 95 and it looks like a change. Then he gets 100 and it looks like a million. It's really disruptive." Work by Eric Seidman and others on apparent velocity is well in play here, but due to all of these elements, Strasburg's fastball is a devastating weapon.

2. Slider
As good as Strasburg's fastball is and as much as hitters have to sit on it for any chance to get to it, the slider might actually be as good, if not a better, pitch. Scouts told me when they first saw him that while his fastball was awesome, it was the slider that really caught their eye. One gave me the great line that when he first saw it in-game, he thought it had hit something mid-flight. There's no question that the pitch is devastating, a two-plane breaking ball that not only is tough to hit due to the movement, but due to the velocity. The pitch comes in somewhere between 84-89, making it something of a change of pace as well as a breaking ball. Mark Prior used his curve in the same way, using the actual changeup as more of a show-me pitch. Hitters aren't having much luck with it, either. One player that played against him in Double-A asked me not to use his name "because he embarrassed me. I had no chance against that guy. He buzzed a fastball down the middle that I barely saw, then threw a slider that broke a mile. When it caught the corner, I just hacked at strike three." 

Strasburg's slider has been compared to one that surprised me. "Jonathon Broxton," an NL scout told me last week. "It's fast, it's based off his heat, and it's really sharp." Pitch-f/x says that Broxton's slider moves in two distinct ways—eight inches right to left and seven inches down from the expected plane. (The drop is actually more severe, but I'll leave the explanation of that to one of the sharper knives here.) The pitch can very literally be heading for the batter's hip and then break enough to catch the back of the plate. Worse, the movement is so severe for both Broxton and Strasburg that the umpire can have a hard time. "You saw this more in college with [Strasburg]," the scout told me, "but umps would get crossed up. Some wouldn't believe it could break that much, and others figured it probably broke enough to catch the plate if it was a strike." Watch to see where the umpire is setting up and if the catcher (expected to be Ivan Rodriguez*) can help him out with some framing.

* Rodriguez is coming back from a quick rehab assignment in order to catch Strasburg's debut. 19 years ago, in his second major-league game, Rodriguez caught Nolan Ryan. That's a pretty nice bookend for a Hall of Fame career.

3. Mechanics
Let's be clear—we don't know the forces that Strasburg's delivery is putting on his arm. We don't know how he feels before, during, or after each pitch, each start. I don't know it, you don't know it, and sadly the Nats don't know it, either. That said, we can take a look at a couple visible things that will give us a clue. First, Strasburg's mechanics are both smooth and repeatable. This is very apparent with even a quick glimpse of his pitching. He's not a max-effort guy even when getting into triple digits, and all of his pitches look very similar. Nowhere is this more clear than in his warm-ups. Whether he's in the pen before a game or on the mound getting ready for an inning, you'll see what one scout called "digital copies." He's a tall pitcher and he pitches tall. (Yes, I know that sounds goofy, but go with it.) He has a very stiff leg and he "flamingos" at the top of his windup. His timing is very good, his tempo fast but not rushed, and he gets his elbow up. 

As he goes into the acceleration phase, many will point at the "Inverted W" that's created by Strasburg's elbows. That's actually a scapular retraction, something you see in some pretty healthy pitchers. Pitching coaches seem less focused on that type of figure and more about timing. Rick Peterson often discusses timing between the front foot landing and the ball being in the "high-cocked" position. Strasburg has that, and you can see it if we get the right angles on replays. Trust me, don't try it from the normal center-field camera angle. As Strasburg accelerates through his delivery, he "stacks up" very well. That means that his foot, knee, glove, chest, and head are lined up over a theoretical pivot point. It's clearly demonstrated in this picture. You'll also see in that picture that Strasburg has very loose, fluid wrists. It might be easier to see that his abduction angle is right. Strasburg doesn't have much shoulder tilt, and the angle created by his body and upper arm is often apparently right on 90 degrees. Like everything else in his delivery, the follow-through is very smooth and consistent. He doesn't end up in the best defensive position ever, but so few have hit it back at him that it's understandable. No one seems concerned about his ability to play his position, and he's more athletic than many expect. 

4. Stamina
It's going to be very difficult to get any real gauge on Strasburg's stamina in this start or for much of the rest of the year due to the limits the Nats (and Scott Boras) are putting on the kid. That makes this start a bit more important. Let's assume that Strasburg is human and has some serious adrenaline going when he gets the big stadium and pro hitters at the plate. If he's able to control that and "stay within himself," that's a big plus. If he comes out playing to the crowd and the radar gun, we might see him tire a little bit as he approaches 60 pitches. Strasburg doesn't often allow himself to "hulk up," as one scout described Aroldis Chapman's style recently. Chapman, a much less polished if nearly as talented pitcher compared to Strasburg, often amps up his fastball to the upper 90s when he's frustrated or feels he needs a pitch or out.

Look for any sign of fatigue in his velocity chart. There should be a slow but steady decline after the sixty-pitch mark with the occasional bump. This is a very normal pattern, though it should be more apparent with Strasburg. Due to the increased top end of his pitches, the physicality of his effort makes a bigger jump than normal possible. Where the average pitcher lives around 90 and can touch 94, Strasburg lives at 94 and can touch 102. Showing the type of math skills that makes Baseball Prospectus famous, eight is more than four, and even more, it's more noticeable at the higher level. It's unlikely that Strasburg will be allowed to go much beyond 80 pitches in his first start and will be both pitch- and innings-limited over the course of the season. One athletic trainer wondered if he'd adjust well to the humidity of Washington: "He's lived in paradise his whole life, with perfect weather year-round. We really don't know much about his conditioning or his acclimatization." 

5. Composure
As I said above in the stamina section, Strasburg's demeanor on the mound has been described as everything from "controlled" to "robotic" to "aloof." Observers can see what they want to see in his dispassionate exterior, but he never seems to lose his composure on the mound. One FOT mentioned this as a major point of concern for him after his recent start against Toledo. "There were some professional hitters there, like Carlos Guillen," he told me, "and I wanted to see if a couple good hits off good pitches shook him up any, if he lost any confidence or started to lose his sequencing. He didn't." Some scouts have been a bit concerned by this, spouting the old saw that a player has to deal with failure before he can succeed. Much like Mark Prior's march through the minors, Strasburg wasn't challenged much. A hit here or there isn't going to tell us much. With Prior, he never struggled until after his injury, and he never could get back through it, with many thinking that his stoic attitude contributed to that. There are some comparisons here. "They're both San Diego kids," said one scout who saw both pitch in college, "and both were men among boys. Both had to deal with the pressure of being stars before they were picked, big contracts and all that. I think the influence of Tony Gwynn is one factor in [Strasburg's] favor. Gwynn always preaches not too high, not too low, and I think it took pretty well on this kid." 

 With the advent of MLB Network and the timing of Strasburg's debut with the 2010 draft, the stars have aligned for what has to be the most-watched debut of all time. I've given you five things to watch, but let me add in one more, perhaps the most important one. Watch the hitters. See if their knees buckle. Look to see if they shuffle their feet between pitches or move back a bit in the box. Are they getting good swings or flailing at where they think the ball might be? The most important thing we might see tonight is whether the hitters are talking about Strasburg to their teammates in the dugout or telling the first baseman that his pitcher is all hype.