The clinical microbiology laboratory includes bacteriology, mycobacteriology, virology, mycology, and parasitology and therefore plays a central role in the diagnosis and management of infectious diseases. Specimen collection is perhaps the most important aspect of infectious disease testing. Failure to collect quality samples can lead to unnecessary delays and/or misleading results. Microorganisms may be detected directly from clinical specimens microscopically or by demonstrating the presence of their antigens or nucleic acids, but culture remains essential due to its broad applicability and the need for isolates to perform complete antimicrobial susceptibility testing. Once isolated in culture, microorganisms may be identified using a variety of approaches with newer molecular technologies increasingly replacing traditional biochemical and antigen testing. In vitro antimicrobial susceptibility tests are performed routinely for most bacterial and yeast pathogens, but such testing for anaerobes, molds, and most mycobacteria is typically performed only upon request. The scope of clinical microbiology is extensive. This chapter provides a very broad overview of infectious disease diagnostics meant to highlight the concepts that are most important to the clinical team.