This thesis investigates control infinitives and ECM-infinitives in the history of Swedish. Both constructions are non-finite, based on infinitives with or without complements, but share some properties and functions with finite subordinate clauses. Control infinitives (to-infinitives) are headed by the infinitive marker att (which in some cases may be omitted) and have invisible PRO-subjects (“controlled” by, i.e. co-referential with, the subject or object of the matrix), whereas ECM-infinitives are headed by overt subjects, distinguished by their “exceptional case marking” (ECM) from the matrix verb, and never contain the infinitive marker. According to the proposed analyses, conducted within the theoretical framework of generative grammar, control infinitives are CPs, taking the infinitive marker as a non-finite complementizer in C, but lack the TP of the I-domain, whereas ECM-infinitives have no C-layer but, nevertheless, a (sort of) TP. The historical investigation shows that control infinitives have developed more clause like properties over time. In Old Swedish (1220–1526), they only rarely contained e.g. negations or auxiliaries. It is not until the seventeenth century that these elements have come into use in the same way as in modern Swedish. This is accounted for by assuming that the control infinitive in Old Swedish was a recent innovation that did not initially make any use at all of the I-domain. The ECM-infinitives, on the other hand, are taken to have the same structure and function in Old Swedish as in Modern Swedish, as their use and properties have not changed significantly. In addition, the status of the infinitive marker has changed through the history of Swedish. Etymologically a preposition, it is here analysed as a verb phrase element in Early Old Swedish, not as a (non-finite) complementizer as in Modern Swedish. In early Modern Swedish (1526–1732), the preposition till is used in much the same function as att giving rise to two new infinitive markers: till att and till. This development of new infinitive markers is also accounted for in the thesis.