Coordinate structures have traditionally been analyzed as having no internal structure other than the sequencing of their members. In particular, the possibility that the members of coordinate structures may themselves be coordinate structures has not been widely recognized. Those who have recognized the possibility of such embedding of coordinate structures have assumed that there are no limits on the depth of such embedding, just as there are no limits on the depth of embedding in subordinate structures. However, coordinate-structure embedding in English occurs only in order to prevent coordinate structures from containing distinct connectives (e.g., and and or), distinct junctures (breaks) between members, and sequences of members in which the first is introduced by a connective while the second is not. In order to prevent these conditions from arising, the depth of coordinate-structure embedding does not have to exceed 2. This limitation on coordinate-structure embedding must be dealt with by the grammars of natural languages; it is not simply a performance limitation. Standard generative mechanisms (finite-state, phrase-structure, and transformational grammars, including recently proposed analyses of coordinate structures based on the minimalist program) do not provide an adequate account of this limitation. On the other hand, a theory of constraint satisfaction such as optimality theory, in which ranked constraints select the optimally structured outputs for given inputs made up of members, connectives, and junctures, does do so. A detailed optimality theoretic analysis of coordinate structures in English is proposed which accounts for the limitation on coordinate-structure embedding, as well as several other properties of those structures, and of their interactions with subordinate constructions.