Increasing data demonstrate that cellular cross-contamination, misidentified cell lines, and the use of cultures at high-passage levels contribute to the generation of erroneous and misleading results as well as wasted research funds. Contamination of cell lines by other lines has been recognized and documented back to the 1950s. Based on submissions to major cell repositories in the last decade, it is estimated that between 18% and 36% of cell lines may be contaminated or misidentified. More recently, problems surrounding practices of over-subculturing cells are being identified. As a result of selective pressures and genetic drift, cell lines, when kept in culture too long, exhibit reduced or altered key functions and often no longer represent reliable models of their original source material. A review of papers showing significant experimental variances between low- and high-passage cell culture numbers, as well as contaminated lines, makes a strong case for using verified, tested cell lines at low- or defined passage numbers. In the absence of cell culture guidelines, mandates from the National Institutes of Health (NIH) and other funding agencies or journal requirements, it becomes the responsibility of the scientific community to perform due diligence to ensure the integrity of cell cultures used in research.