This detailed study challenges the claim that syntax is arbitrary and autonomous, as well as the assumption that Spanish clitic clusters constitute grammaticalized units. Diverse--apparently unrelated--restrictions on clitic clustering in both simplex VP's and Accusative cum Infinitive structures are shown to be cognitively motivated, given the meaning of the individual clitics, and the compositional/interpretative routines those meanings motivate. The analysis accounts, in coherent and principled fashion, for the absolute non-occurrence of some clusters, and the interpretation-dependent acceptability of all remaining clitic combinations: cluster acceptability depends on the ease with which the given clitic combination can be processed to yield a congruent message; there is no point in combining clitics whose meanings preclude speedy processing of the cluster. The monograph goes beyond previous work on Spanish clitics in its wealth of data, the range of syntactic phenomena discussed, and its analytic scope.