The exact nature of his current illness has not been revealed, but he will undergo tests and remain in hospital for a number of days.
Assistant manager Gary McAllister will take charge of Villa's home clash with Stoke at the weekend.
A Villa statement read: "The club can confirm that Gerard Houllier was admitted to hospital.
"He is in a comfortable condition and he wishes to convey his appreciation to Aston Villa supporters for their concern and good wishes.
"The manager spoke to chief executive Paul Faulkner this morning and he is currently undergoing tests. It is anticipated that he will remain in hospital for several days."
Source: PA
Source: PA