A serve is legal when the ball starts still on your open palm, is tossed up at least 16cm without spin, is hit as it falls from behind the end of the table, bounces once on your side and once on your opponent's, and stays in the receiver's sight the whole time. Break any of those and you lose the point. The wording comes from law 2.6 of the Laws of Table Tennis, written by the International Table Tennis Federation (ITTF) and used in England in the version published by Table Tennis England.
Those rules exist because the serve is the one shot your opponent can't influence. A server who could hide the ball or flick it from the hand would win most points outright. Each part of the law closes off one way of doing that.
Start with the ball still on an open palm
The serve begins with the ball "resting freely on the open palm" of your free hand, which is the hand not holding the bat. Your hand must be stationary. In practice that means a flat hand, fingers together, thumb free, with the ball sitting on top rather than cupped or pinched.
Cupping the ball is the most common fault in club matches, often without the server knowing. It matters because a ball held in the fingers can be spun or thrown at an angle on the way up.
Toss it at least 16cm, almost straight up
You must project the ball "near vertically upwards, without imparting spin" so that it rises at least 16cm after leaving your palm. That is a little more than the height of the net, which is 15.25cm. The ball must then fall without touching anything before you hit it.
There is no maximum height. Some players toss the ball high above their head to add speed and spin as it drops, which is legal as long as the toss is near vertical.
Hit it on the way down
You must strike the ball as it is falling, not at the top of the toss or on the way up. It must bounce first on your side of the table and then go directly to the receiver's side. If it misses your side, or lands on your side twice, the serve is a fault and the point goes to the receiver.
In singles you can serve to any part of your opponent's court, short or long, to the forehand or backhand. In doubles the serve must go diagonally, from your right half of the table to the receiver's right half. The white centre line counts as part of each right half. Doubles explained has the rest of the doubles order.
Behind the table, above it, and in full view
From the moment the ball leaves your palm until you hit it, the ball must be above the level of the playing surface and behind your end line. The laws treat the end line as extending indefinitely in both directions, so you can stand wide of the table to serve, but you can't lean over it and hit the ball above the table.
The ball must not be hidden from the receiver by you, by your doubles partner, or by anything either of you wears or carries. As soon as you have tossed the ball, your free arm and hand must move out of the space between the ball and the net. This is the hidden-serve rule. It stops a server screening the moment of contact with an arm or shoulder, which would leave the receiver unable to see which way the bat moved and guessing at the spin.
Your body can still turn and your bat can move fast and in any direction. Disguise through bat speed and fake motions is allowed. Disguise by blocking the receiver's view is not.
Lets: when you serve again
If your serve touches the net, the post or the net clamp and then lands correctly on the receiver's side, it is a let and you serve again with no penalty. There is no limit on how many lets you can serve in a row. If the serve clips the net and lands anywhere else, it is a fault.
A serve is also a let if you serve before the receiver is ready, as long as neither the receiver nor their partner tries to hit it. If they swing at the ball, they are treated as ready.
What happens with a doubtful serve
The law puts the burden on the server. You must serve so that the umpire or assistant umpire can be satisfied it is legal, and either can call it incorrect. If an official isn't sure, they may, on the first occasion in a match, stop play and warn you without losing the point. After that, any serve by you or your doubles partner that is not clearly legal loses the point.
So a warning is a signal to toss higher, flatten your palm and move your free arm away quickly. You don't get a second one.
In most local league matches there is no umpire, and the players manage the serve themselves. If you think an opponent's serve is illegal, the usual course is to mention it politely between points rather than stop a rally. Our guide to the rules explains how club matches are run.
Players with a disability
The umpire may relax the serve rules for a player whose physical disability prevents them from complying, for example someone who can't toss the ball from an open palm. Wheelchair table tennis also has its own let rules for serves that bounce back towards the net or stop on the receiver's side.
Next steps
The fastest way to learn a legal serve is to practise the toss on its own: flat palm, still hand, up past net height, hit on the way down. Once that is automatic, you can start adding spin with the bat, which our guide to topspin, backspin and sidespin covers, and thinking about where to put it, which is the subject of serve and receive tactics.