A frequently mentioned advantage of CLIL is that it is a theory of practice, with an intrinsic flexibility and heterogeneity; nonetheless, there is often confusion regarding the factors of success and failure among teachers applying the approach, which often leads them to unwittingly misapply CLIL.This article first presents several definitions of CLIL, and the practical and theoretical assumptions at its core, and then analyzes a number of longitudinal studies from such places as Canada, the Netherlands, Spain, Hong Kong, the Philippines, and Malaysia in order to identify factors that contribute to the relative success or failure of this methodological approach.Drawing on a basis of research and training courses for CLIL teachers, this study presents its unique and defining factors and presents guidelines for teachers who wish to apply it.