This book presents an analysis of Spanish prepositional clauses (< P + CP >) - complement and adverbial clauses. The goal is to examine the syntax and evolution of those clauses and their components in Spanish, contrasting them with other European languages. Prepositional argument and adjunct clauses are grammatical in present-day Spanish. However, Medieval Spanish only attests the latter; the former were not frequent until the 16th/17th centuries. Both types are examined in their syntactic evolution and properties, including clausal nominality, argumenthood, nature of prepositions, and optionality. Latin and Portuguese, French, and Italian - both in their present-day and past forms - are studied and compared to Spanish. Likewise, several Germanic languages are surveyed. These languages show variable grammatical degrees of < P + CP >. The comparison reveals aspects which challenge the commonly accepted conclusions about the clausal patterns of each language. This study offers a novel approach to the analysis of Spanish prepositional clauses by looking at its properties and formation not only from within but also in contrast with other languages. It argues for cross-linguistically valid categories and explanations in order to comprehend the properties of human language.