Understanding software is getting more complex as programs are getting ever bigger. Many believe that the key to this problem is better visualization. The objective of this document is to provide an as thorough as possible overview of this field. This is the starting point for further research and development in this critical area. Over 140 papers and nearly as many tools were reviewed for this purpose. This document presents the most relevant and significant ones. It also tries to identify the essential requirements of a software visualization component.