Human cytomegalovirus (HCMV) is associated with several diseases in immunocompromised individuals. CMV infection can be diagnosed directly by demonstration of the virus or virus components in pathological materials or indirectly through serology. Molecular biology has allowed detailed studies of the viral genome and its antigenic gene products and has led to major advances in CMV diagnosis providing new tools for both the analysis of the CMV-specific immune response and the detection of virus-specific antigens or genetic material. In an attempt to provide a guide for the correlation between laboratory findings and clinical interpretation, we discuss in this work the clinical settings in which the presence of CMV needs to be diagnosed, and how a CMV diagnosis should be asked for or interpreted by the clinician in view of the variety of new or improved laboratory tests now available.