In this paper, I review how formal features are currently regarded and used in the Minimalist Program. Although features are the cornerstone in Minimalism, they are used in many different and conflicting ways. Features may seem particularly relevant to affix-hop because the affix has to be checked against a higher verb or auxiliary. Chomsky’s (1957) analysis of affix-hop has the affix connected with an auxiliary, e.g., the -en of have-en, move to a verb on its right, as in have see-en. This analysis is one of the high points of early generative grammar but, with each new instantiation of the generative model, it has needed adjustments and the phenomenon is still debated. I will elaborate on a proposal made in van Gelderen (2013) who argues that interpretable tense, mood, or aspect are in a low position being probed by the relevant uninterpretable features in a high position. This view I claim is consistent with data from change and acquisition. I also discuss the implications of this reliance on features for learnability and Universal Grammar.