While cell replacement offers hope for the treatment of many diseases in the long term, it may still be some time before large-scele clinical use is available for most applications. Understanding how to produce many of the speialized cel types in vitro remains a major hurdle. Furthermore, the field faces challenges around quality control. It is essential that only defined cell populations are introduced into patients; this requires careful characterization of the cell populations intended for transplantation, in terms of gene expression and epigenetic profiles and functional attributes, and also to ensure that the populations do not contain other potentially harmful cell types. For cells generated from human pluripotent cells, for example, contamination of the transplant population with even a small number of residual embryonic stem cell or induced pluripotent stem cells could promote tumor formation. Additionally, as cells can acquire mutations during the culture process, stringent quality control is essential to ensure that cultured cells intended for transplantation have not acquired undesirable properties.