The Cellularization Principle states that under rather weak conditions a Quillen adjunction of stable model categories induces a Quillen equivalence on cellularizations provided there is a derived equivalence on cells. We give a proof together with a range of examples. The main result here was originally presented as an appendix of arXiv:1101.2511. However, the Cellularization Principle has many other applications which are explored here. The material in arXiv:1101.2511v3 will be revised to refer to this paper.