Laboratories interested in working with in vitro cultured cells must be able to freeze and thaw cell lines. Such capabilities are necessary to maintain the constancy of the cells since prolonged culture may result in phenotypic and genotypic changes. In addition, cell freezing minimizes problems with contamination as well as the overall expense of maintaining cell lines for long periods. Finally, freezing of cells provides a method of making cell lines available to other laboratories since frozen cells are readily shipped to distant points. This appendix describes how cell lines can be frozen for long-term storage and thawed for growth in a tissue culture vessel.