This chapter presents an overview of microprocessor and application specific integrated circuit (IC) testing. It begins with a description of key industry trends that will impact how ICs will be tested in the future. Next, it provides a brief description of the most common tests applied in the IC industry, where technical issues that are causing methodology changes are emphasized. These include functional testing, structural testing, scan-based delay testing, built-in self-testing, memory testing, analog circuit testing, system-on-a-chip testing, and reliability testing. Trends discussed have driven the development of novel focus areas in test and the chapter discusses several of those areas, including test data volume containment, test power containment, and novel methods of defect-based test.