Over the past 40 years, Jim Cummins has proposed a number of highly influential theoretical concepts, including the threshold and interdependence hypotheses and the distinction between conversational fluency and academic language proficiency. In this book, he provides a personal account of how these ideas developed and he examines the credibility of critiques they have generated, using the criteria of empirical adequacy, logical coherence, and consequential validity. These criteria of theoretical legitimacy are also applied to the evaluation of two different versions of translanguaging theory - Unitary Translanguaging Theory and Crosslinguistic Translanguaging Theory - in a way that significantly clarifies this controversial concept.