Deterministic operation of a microprocessor is highly valuable in the test and debug of silicon devices, as well as in some specialized system applications. There are a number of design, verification, and test challenges in creating a microprocessor that can run deterministically. This paper will discuss actions needed to achieve this and will describe of variety of real-world issues that have been encountered and addressed over the course of some recent microprocessor designs. Challenges for future designs will also be briefly discussed.