The study is based mainly on data from the Early Modern English Section (1500 - 1710) of the Helsinki Corpus of English Texts. The dissertation consists of six chapters. Chapter 1 is an introduction to the aims, subject, data and methodology of the study. Chapter 2 is a blief survey of the treatment of the conditional sentence in early English grammars. Chapter 3 demonstrates that the grammaticalization of conditional markers is motivated by the need to express either a supposition or a kind of contingency relation between the protasis event and the apodosis event. It identifies the following as the sources of protasis markers in EModE: (a) suppositional verbs and verb phrases; (b) verb forms and word order that signify or imply hypotheticality; (c) expressions signifying contingency relation; (d) expressions signifying other relations, including temporal, provisory, purposive or resultative, exceptive, and coordinative; (e) expressions of negative substitution. Chapter 4 shows that markers of concessive condition are not fully grammaticalized in EModE. In most cases concessive conditions are expressed either by conditional markers used in a concessive context or by concessive markers used in a hypothetical context. Chapter 5 distinguishes four kinds of hypotheticality -- low, neutral, high, and counterfactual -- and studies the ways in which each kind is encoded in EModE. It reveals a number of constraints on the choice of verb fo1ms in protases with presentor future-time reference and with neutral and low hypotheticality. Chapter 6 postulates that the meaning distinctions in te1ms of the conditionality and hypotheticality identified in this study are of a universal nature and that the grammaticalization of these meaning distinctions is in the direction of their full and precise expression but is constrained by pragmatic considerations. It suggests that the case of conditional sentences poses some interesting problems for a general theory of grammaticalization.