In this paper we show that focus structure determines the interpretation of bare plurals in English: topic bare plurals are interpreted generically, focused bare plurals are interpreted existentially. When bare plurals are topics they must be specific, i.e. they refer to kinds. After type-shifting they introduce variables which can be bound by the generic quantifier, yielding characterizing generics. Existentially interpreted bare plurals are not variables, but denote properties that are incorporated into the predicate. The type of predicate determines the interpretation of its bare plural subject. The individual/stage-level distinction, though important, is not sufficient: since only arguments can be topics, only those stage-level predicates which have locative arguments can have existential bare plural subjects. Certain verbs (e.g., hate) fail to incorporate their bare plural objects; therefore no existential reading of the object is available. We provide a novel solution to this puzzle based on the following two claims: (i) incorporated bare plurals do not introduce discourse referents; (ii) nonincorporating verbs are presuppositional.