Given an integer matrix A, there is a unique matrix S of a particular form, called the Smith Normal Form, and non-unique unimodular matrices P and Q such that PAQ = S. It is often the case that these matrices P and Q will be used for further calculation, and as such it is desirable to find P and Q with small entries. In this thesis we address the problem of finding such P and Q with small entries, in particular in the case where A is a diagonal matrix, which arises as a final step in many published algorithms. Heuristic algorithms are developed which appear to do well in practice and some theory is developed to explain this behaviour. We also give an account of the implementation of an alternative algorithm which bypasses this intermediary diagonal form. The basic theoretical development of this is work by Storjohan.