The study of clitic clusters and the restrictions that surface when two or more clitics are combined have long intrigued linguists and, as such, clitic phenomena are at the core of an ever-growing body of research in linguistic theory. However, three aspects remain largely unexplored when it comes to clitic cluster constraints, namely the evolution of these restrictions through time, the perception and processing of different clitic combinations, both acceptable and unacceptable, by native speakers and the acquisition of such combinations by non-native speakers. This dissertation, which focuses on 1st and 2nd person clitic clusters in Spanish, aims to shed new light on clitic phenomena with a new analysis and new data from all these perspectives. Specifically, I study the effects that case and marked features have on Spanish clitic combinations, both synchronically and diachronically. In addition, I explore the effects of clitic combination restrictions in language processing and analyze the learnability issues derived from such restrictions in three groups of speakers of Spanish as a second language whose L1s are English, French and Romanian, respectively. At a particular level, this dissertation is a study of clitic cluster constraints from different perspectives, both traditional and new, namely linguistic theory, diachrony, language processing and language acquisition. At a general level, it constitutes an attempt to explore the ways in which linguistic theory can guide applied research and, conversely, the ways in which experimental data may contribute to linguistic theory.