This paper reports a critical comparative analysis of two popular and significant theories of adult learning: the transformation and the deep approach theories of learning. These theories are operative in different educational sectors, are significant, respectively, in each, and they may be seen as both touching on similar concerns with learning that is profound in its nature and impact on the learner--hence the case for a critical comparison. The critical analysis focused on similarities and differences between the two theories on a set of general criteria. It found that, while there are unacknowledged similarities, the differences are complementary, each theory suggesting a different way of considering the same territory, without excluding the other theory. The analysis strongly suggests the imperative for research findings from each theory to be used to inform practice and research through the other, although the literature reveals a lack of such cross-fertilization.