What command to see which file is opened by a program during execution?

[VJ]

Hi,
   I forgot this command which could be used to see what files are
accessed by a program during it's execution time, the syntax was like
following

command_i_forgot program_i_want_to_monitor

and it used to print all files/shared libraries a program used to load
while program was running.

Can anybody tell what command is that?

Regards from

VJ