I present a brief introduction to contrastive hierarchy theory, aka Modified Contrastive Specification (MCS) or 'Toronto School' phonology. I set out the main tenets of this theory, and consider what implications the theory has for our understanding of phonological features. I argue that the language learner's task is to arrive at a set of ordered contrastive features that account for the phonological patterning of the input language. This requirement puts strong constraints on phonological representations, and accounts for why phonological systems resemble each other, even without assuming that features are innate. For example, any three-vowel system allows exactly two contrastive features; four-vowel systems may have two features if symmetrical, or maximally three if not. This approach also makes it possible for us to consider possible parallels between phonological and morphosyntactic features in a new light.