By the 1970s, Knud Illeris was well known in Scandinavia for his developing work on project studies in theory and practice. In this work, learning theory was applied, mainly as a combination of Jean Piaget’s approach, Carl Rogers’ ideas of significant learning and the understanding of late modern qualification needs as developed inside the so-called ‘critical theory’ of the German-American Frankfurt School. After several years of practical developmental work in youth and adult education and working life, Illeris returned to his learning theoretical roots during the 1990s, now involving many other theoretical approaches in building up a general and comprehensive understanding of learning, which was first presented in his book, The Three Dimensions of Learning in Danish in 1999 and in English in 2002. This was then in 2006/2007 fully worked out in How We Learn: Learning and Non-learning in School and Beyond, and completed by the new edition of this in 2015/2017. The following chapter presents the main ideas of Illeris’ understanding and is an elaborated version of a presentation given at a conference in Copenhagen in 2006.