This paper proposes an analysis of Mandarin Chinese negation in whichbu ‘not’ is an adverb in Spec,AuxP or Spec, VP which may occur only with unbounded aspectual situations and which must cliticize to the following element at S-structure. Evidence comes primarily from the facts that (1)bu does not normally occur with perfective aspect markers or with telic situations (such as achievements), and (2) whilebu occurs with preverbal adjuncts, it cannot cooccur with certain types of postverbal adjuncts: it is argued that postposing the latter from their base preverbal position leaves a trace which preventsbu from cliticizing, as it must. Evidence comes also from (3) the untenability of alternative analyses invoking scope or overt head movement. This analysis supports the views that morphological requirements of functional items may have important consequences for syntax and that the distribution of such items can be accounted for, in part, by ‘Affix Checking’, i.e., head movement at LF rather than in overt syntax.