The purpose of this study is to propose a lexical analysis of inflection and cliticization in Spanish within the framework of Categorial Grammar, and to show how morphology and syntax interplay with one another in this language. I postulate that inflectional suffixes and clitics are syntactic expressions in their own right; inflectional suffixes are the instantiation of the grammatical relation, subject, whereas certain clitics, i.e. DAT and ACC clitics, are of the object. In this regard, inflection and cliticization can be conceived as functions from one set of syntactic expressions into another. I assume that inflectional suffixes and clitics are stored in the lexicon assigned to categories which specify their syntactic (and semantic) properties. These elements are combined to form complex expressions by two kinds of operations: (1) Function/argument application, and (2) Functional Composition. Three lexical rules are proposed in order to account for the distribution of the morphological properties at issue: (1) Inflection, (2) Cliticization, and (3) Complex Verb Formation. These rules make an explicit statement of what syntactic processes take place as morphologically complex expressions are formed. One consequence of my analysis is the redefinition of nominals commonly referred to as "subject NP" and "object NP" (doubled by a clitic) as elements which mark a referential contrast. This way, the formal variation as to the presence or absence of these nominals in Spanish sentences has a coherent explanation. Two rules of nominal adjunction are proposed in order to account for "clitic doubling" and "subject doubling". These two rules apply under certain conditions. With a lexical treatment of inflection and cliticization proposed, all the word formation processes in the Spanish language are now relegated to one single component, the lexicon. Morphology in Spanish, thus, has a clearly delineated domain of its own as an integral part of the lexicon. Furthermore, inflection and cliticization are morphological processes which, at the same time, construct syntactically complex expressions. This direct interplay between morphology and syntax is what uniquely characterizes the so-called "pro-drop" languages, of which Spanish is one, and distinguishes them from the "non-pro-drop" languages.