Houllier had heart problems that had kept him out for the end of last season, and has now had to leave the post after undergoing tests earlier on in the week. From the results, the doctors advised that he would need further time to recover and so would not be able to be involved in the stress of pre-season.
Aston Villa asked him to resign because they felt that going back to the dug out after pre-season would have a major risk on bringing his heart problems back which they did not want to be responsible for.
In 2001 Houllier had a heart operation after he experienced chest pains during a game and that time he managed to come back and remain Liverpool manager for another threee years.
Although Villa did not have a very good season, experiencing some relegation worrys. Houllier managed to turn that around with his aquisition of Darren Bent for £18million from Sunderland.
Houllier had been hoping to keep his job however despite resigning:
"I am extremely disappointed that I will not have the opportunity to manage Aston Villa next season.
"My health has improved considerably since I was taken ill on 20 April. I now intend to take the next few months to concentrate on recuperating fully before I may return to football.
"I was very much looking forward to the prospect of my first full season as Villa manager and returning for pre-season training.
He added: "It has been an honour to manage this great club and I wish everyone connected with Aston Villa every success for the future."