Active HCMV infection is diagnosed by isolation of the virus or by demonstrating its presence of viral genome or viral antigens. The viral genome can be directly detected by molecular biological techniques. The two most popular techniques are polymerase chain reaction (PCR) and in situ hybridization (ISH). The sensitivity of PCR is superior to the detection of viral antigens in infected cells or isolation of viruses in culture. HCMV-DNA in peripheral leukocytes is often positive in healthy people. To discriminate a latent infection from an active infection may be done by reducing the number of amplification cycles, by testing in cell free blood samples (serum or plasma), by amplification of viral mRNA transcripts, or by quantification of HCMV DNA. ISH is the method for diagnosis by demonstrating the viral genome in cells of involved organs and is more sensitive and specific than histology or immunohistochemistry. The viral genome can be detected even in formalin fixed paraffin-embedded sections. In situ PCR (IS-PCR) is a new molecular technique, that combines the extreme sensitivity of the PCR with the cellular localization provided by ISH, and that can detect even one copy of viral DNA per cell. And also other procedures for quantitative HCMV DNA detection, Hybrid Capture system, and serodiagnosis using the recombinant antigens are described.