When a patient is being evaluated for infection, a thorough history and physical examination, microbiological assessment, and other diagnostic tests usually are required. In addition to microbiological evaluations, the clinical laboratory can provide other diagnostic measurements that may help to diagnose/identify the infection or to evaluate the stage of an infectious disease or process. This chapter discusses laboratory test methodologies used in the detection, treatment, interpretation, and monitoring of infections. Although this chapter is not all inclusive, it will offer the infection preventionist (IP) a better understanding of the type and scope of testing that is available from the clinical laboratory.