The detection of mycoplasmas in human and animal cell cultures is mandatory for every cell culture laboratory, because these bacteria are common contaminants, persist unrecognized in cell cultures for many years, and affect research results as well as the purity of cell culture products. The reliability of the mycoplasma detection depends on the sensitivity and specificity of the method and should also be convenient to be included in the basic routine of cell culture quality assessment. Polymerase chain reaction (PCR) detection is one of the acknowledged methodologies to detect mycoplasmas in cell cultures and cell culture products. Although the PCR offers a fast and simple technique to detect mycoplasmas, the method is also susceptible to errors and can produce false positive as well as false-negative results. Thus, the establishment and the routine application of the PCR assay require optimization and the inclusion of the appropriate control reactions. The presented protocol describes sample preparation, DNA extraction, PCR run, the analysis of the PCR products, and speciation of the contaminant. It also provides detailed information on how to avoid artifacts produced by the method. Established properly, PCR is a reliable, fast, and sensitive method and should be applied regularly to monitor the contamination status of cell cultures.