||During the last century, the field of toxicology has evolved from one of describing the consequences of exposure to chemicals, to xenobiotics in animals, to mechanistically-based studies of both animals and cell or tissue cultures. During the last decade, in vitro studies of cell and tissue cultures, such as the Ames Mutagenicity Assay, have become an important approach used to understand the consequences of exposure and to assign risk. In both U.S. and European laboratories, scientists have vigorously pursued the development of in vitro methods to advance their science. During the last 20 years, the considerable and significant advances in tissue culture methodology, the use of chemically-defined cell and tissue culture media, and the availability of human cells have transformed in vitro methods from a new technology to a valuable research tool. In product development, drug discovery, and safety evaluation, the use of in vitro tests has become commonplace, resulting almost exclusively from the evolution of science rather than any fundamental change in philosophy. Yet all in vitro methods are alternatives to animal testing.