Fastest Way to compute a factorial. . .

I have been trying to work it out for a long time now. Many of the programming challenges involve frequent us of factorials which contribute to the solution of the problem. Needless to say, the value gets large fairly quickly, and gets difficult to calculate in small time slice too.

So can any of you explain to me an efficient way of finding factorials. It would be of great help!

Thanks in advance,

Osman :)