Matrix Equation Help?

Dear R-listers

Now I have a Matrix Equation to solve,
A -- m by n matrix, where n>>m, known matrix;
X -- n by m unknown matrix;
B -- m by m known matrix.

So given the equation A*X=B, how to solve X?
I first tried to take X = inv((A'*A))*A'. But since
A'*A is singular, this is no way to go.

Please give me some point on this.
Thanks

Fred