In the original Transformers series (i.e. not Beast Wars), Megatron was the leader of the Decepticons. He transformed from giant robot to a smaller-sized Walther P-38 hand gun.

In robot mode, Megatron has a giant cannon on his right arm, which would fire a deadly blast. But he was just as deadly when transformed into a gun; many times, Megatron would transform into a gun, and another Decepticon (usually Starscream) would fire him.

Ruthless and cunning, Megatron was sometimes referred to as "The Slag Maker" because of his destructive abilities. Megatron is also a military genius. Very few can match Megatron's physical strength and intelligence; one of the few equals of Megatron is Autobot leader Optimus Prime.

In the series, Megatron wanted to comquer the planet Cybertron, and destroy any Autobot who stood in his way. He also wanted to tap the planet earth for all its natural energy resources. While he has come close, Megatron has never been victorious in either.

In Transformers the Movie, Megatron and Optimus Prime fought to the death. While some would argue that Megatron won that battle, it was only because he was using Autobot Hot Rod as a shield, so that Optimus wouldn't fire back. Later, Starscream jettisoned Megatron off of their escape shuttle, so that he could become Decepticon leader.

But Megatron, who still had life in him, was re-built by the giant transforming planet Unicron into a meaner, crazier robot named Galvatron. Galvatron would kill Starscream and become Decepticon leader again.

Note: In the Transformers Generation 2 toy line and comic book, Megatron was rebuilt by Cobra (the terrorist organization in the GI Joe series). Megatron's new transformation was into a green and purple military tank. He did not appear like this in the Transformers Generation 2 cartoon, since it was just the old cartoons with a new beginning title sequence.

However, this tank-Megatron appears in the Japanese Beast Wars II cartoon as the Destron Megastorm--Galvatron's little brother. (It's true; ask robotech_master if you don't believe me)