With over 3000 cancer cell lines described in the literature and thousands in regular use, it has become essential to characterize and authenticate cell-line models. Characterization of the properties of cell lines is important for a number of reasons. First, the relationship of the cell line to the cells of origin should be established to confirm that the cell line is derived from and is representative of its tissue of origin. If the cell line is to have any value as a model it should reflect the properties of the cell type from which it was derived. For example, for a cell line established from a breast carcinoma it is helpful to show that the cell line has characteristics consistent with breast and epithelial origin. Although the genetic profile should remain constant, expression may change and features such as differentiation characteristics may be lost over time in culture. Similarly, as the culture develops, certain clones may emerge with selection and predominate. Particularly important is the need to check for purity and potential cross-contamination with other cell lines. The history of cell culture indicates that cross-contamination between cell lines is widely prevalent and continues to be an ongoing problem (1–3). During the 1970s and 1980s, multiple studies initiated by Stanley Gartler and Walter Nelson-Rees demonstrated that one in three cell lines were either contaminated or even totally replaced by other cell lines (4–8). The most frequent contaminant was the HeLa cervical carcinoma cell line which had been established in 1951 (9) and had been widely distributed to many research laboratories. As a result of its rapid growth rate, once mixed with other cell lines it would generally outgrow them. Unfortunately this problem has not disappeared, and contamination continues to be widespread (1,2,10). This area is covered in more detail in Chapter 30.