Human embryonic stem cells (hESCs) and hESC-derived cells are of great interest, not only because of their therapeutic potential but also their prospective uses in in vitro drug and toxicity screening. The ability to preserve these cells is critical, allowing for the generation of quality-controlled stocks of cells, transport of cells between sites, and avoiding the need for expensive and time-consuming continuous culture. Current methodologies, namely conventional slow freezing and vitrification, can successfully preserve hESCs and their differentiated progeny, retaining the key characteristics of the cells. However, there is a significant gap between the number of cells potentially needed to either treat patients or run a high-throughput drug screen and how many cells can be preserved using these techniques. Therefore, this review focuses on the scalability of slow freezing and vitrification, identifying key barriers to success and whether they can be overcome. Given the precedent with other mammalian cells in using slow freezing to successfully preserve large quantities of cells and its compatibility with current and emerging culture methods for hESCs, it is likely to become the method of choice for cryopreserving these cells at scale. However, issues other than scale still exist; therefore, alternatives to cryopreservation should also be explored. Here, the potential to lyophilize hESCs for long-term storage is considered as one such alternative. (C) 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011