Dirty systems are systems which lack documentation, have few or no defined requirements, have no standardized design or architecture, and have unknown levels of defects. These systems are very difficult to maintain and test due to high complexity and low understanding. People are often afraid to make any changes to this type of software because of the high risk of breaking things that currently work correctly. In many cases, these systems are legacy systems and were never tested well originally. After years of maintenance, these systems often are in disarray due to the lack of control. The object of this book is to assist software testers in how to approach testing a system, which in many instances appears as a "black-box" to them. The testing process is heavily focused on a diagnostic step, which scopes the test problem, and assists the tester in building an effective test plan. The process proposed in this book should significantly improve the effectiveness and efficiency of the software testing. There are two main objectives for the reader: 1) learn how to test these complex and large legacy systems, and 2) use the information gained from testing to clean the system and improve maintenance and testing processes.