Literature related to the partitioning of biological materials in aqueous polymer two-phase systems encompasses over two thousand articles and almost half a century of experience (1). Much of this effort has involved eukaryotic cells or other biological particles. Over the past 15 years the methods and results related to these articles have been summarized in a number of books dedicated to partitioning (2-6), as well as in various book chapters, such as ref. 7. In addition, papers from the last three Biannual International Partition Meetings have been published in the Journal of Chromatography (8-10) and a Partitioning “website” exists on the world wide web (11). As such, it is easy to find a plethora of methods and results on the partitioning of various cells (12). What is perhaps harder to come by is appreciation for various experimental factors that influence cell partition. Such appreciation is necessary for researchers who wish to read critically the partition literature, and develop their own partition experiments. A challenge made more difficult because, for a variety of historical and practical reasons, many of the methods used in various aspects of partitioning have not been standardized. This includes methods for phase-system preparation, cell preparation, and the interpretation of results; methods that often involve only minor modification of standard cell biology procedures, or can be found elsewhere in this book. It therefore seems wise that this chapter should not concentrate on one partition method used with one cell type, but rather provide a general introduction followed by a brief outline of protocols, so as to enable the reader to evaluate the literature in relation to their own cell-separation challenge. In addition to ref. 3, which provides a thorough introduction to “Partitioning,” novitiates are encouraged to read other chapters in this book particularly Chapter 1–Chapter 6, as well as those dealing with the fractionation of other particles.