The Megadrive was released in Britain around Christmas in 1989.
Technology wise it was a a bit of a step up from the 16bit computers and the
then most advanced console of the day, the PC Engine.
The Megadrive was an instant hit.
The release of games such as the infamous Sonic the Hedgehog, Streets of Rage
and Revenge of Shinobi cemented it as the games console of choice.
The release of Streetfighter 2 on the SNES was the beginning of a fall
in popularity.
Sega attempted to trump Nintendo with Streetfighter 2 Championship Edition
with a new joypad.
Nintendo immeadiately made them look silly though with the release of
Streetfighter 2 Turbo.
The Megadrive was increasingly looking a bit old, but it continued to sell.
The release of the Mega CD (complete with the dodgy Nightrap,
which encouraged lots of young lads to forget about the game and attempt to
get far enough to see the stripping girls.
Nightrap, surprisingly, was banned) and repackaging as the Megadrive 2
staved off slagging sales.
The release of the Atari Jaguar seems to have surprised Sega, who responded
with the the 32X.
This 32bit expansion, which fits into the cartidge port, was even more poorly
supported than Atari's 'Vunder' console however and it looked a bit pathetic. 

www.zhell.co.uk/megadrve.html