Start a history subsititution, except when followed by a space, tab, or the end of the line... = or (.

!!

Refer to the previous command. This is a synonym for !-1.

!n

Refer to command line, n.

!-n

Refer to a specific command line, specified n lines back.

!string

Refer to the most recent command starting with string.

!?string[?]

Refer to the most recent command containing string.