This dissertation examines prefixation of simplex pairs. A simplex pair consists of an iterative imperfective and a semelfactive perfective verb. When prefixed, both of these verbs are perfective. The prefixed forms derived from semelfactives are labeled single act verbs, while the prefixed forms derived from iterative imperfective simplex verbs are known as distributive verbs. Together, these two prefixed verbs are a perfective doublet. Data from a quantitative analysis of examples from the Czech National Corpus offer insights into the distinction between these verbs. The derivational patterns of perfective doublets are also examined. In many instances both a single act and a distributive verb are derived, while in other instances only one of these types is derived. It is found that these derivational gaps are predictable and based primarily on the spatial semantics of the prefixes. A novel analysis is presented to account for these patterns. Several procedural meanings derived from simplex pairs are also analyzed. It is argued that prefixed verbs in "za-" have a SEGMENTATIVE meaning. They form a natural perfective partner to the simplex imperfective. Based on tests of aspectual pairhood, it is argued that the SEGMENTATIVE procedural verb and the semelfactive simplex together form a type of perfective doublet. The link between the SEGMENTATIVE meaning and the spatial semantics of "za-" is explored. The SEGMENTATIVE meaning is found to be an extension of the spatial meaning of the prefix. Additional procedural meanings are discussed. Verbs with certain procedural meanings are derived from imperfective simplex verbs, while others are derived from semelfactive simplex verbs. The choice of one simplex over another is found to be motivated by the semantics of the procedural meaning. The link between procedural and spatial meanings is also explored for these additional procedural meanings. The findings of this dissertation are shown to be more generally applicable to other derivational processes in Czech, as well as descriptions of aspectual pairhood. Additional paths for studying these verbs are suggested, including types of experimental and historical research. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]