When a patient is evaluated for an infection, a thorough history, physical examination, and diagnostic tests are usually required. In addition to microbiological evaluations, the clinical laboratory can provide other diagnostic measurements that may help to diagnose an infection, identify a pathogen, or evaluate the stage of an infectious process. This chapter discusses laboratory testing methodologies used in the detection, treatment, and monitoring of infections. Diagnostic algorithms are often used to direct diagnostic testing as part of diagnostic stewardship efforts. Testing algorithms ensure high positive predictive values and aim to reduce false-positive and false-negative results, redundant testing, excess treatment, and cost. Although this chapter is not an all-inclusive survey of laboratory diagnostics, it will offer the infection preventionist a better understanding of the types, rigor, and scope of clinical laboratory testing that can aid in diagnosing infectious diseases.