This dissertation concerns the distribution of negative polarity items (henceforth, NPIs) in conditionals and conditional-like constructions. NPIs include words such as any and ever and idioms such as "give a damn" and "lift a finger"; these expressions have only a limited distribution. In this dissertation, the distribution of these expressions in the following three types of conditional and conditional-like constructions are investigated: i) conditionals with possibility modals (e.g., "If John has ever been to Paris, he will become a good chef"), ii) because-sentences (e.g., "John is a good chef because he went to Paris"), and iii) if-only constructions (e.g., "If only John had been to Paris, he would have become a good chef"). Since Klima (1964), NPIs have received much attention in the semantic theory; one of the proposals in current semantic research, which can be traced back to Fauconnier (1975, 1979) and Ladusaw (1979), has suggested that the licensing of these items has to do with downward-entailing-ness (henceforth, DE-ness) in the environments where they occur; to license an NPI, an environment must be a DE-context, where an inference from a set to its subset is supported. In this dissertation, I will show that a naive combination of a DE-based approach of NPI licensing (Fauconnier 1975, 1979; Ladusaw 1979; von Fintel 1999; a.o.) and a Lewis-Kratzer-von Fintel style semantics of conditionals (Lewis 1973a, Kratzer 1981, 1986, 1991; von Fintel 1994; a.o.) fail to predict the distribution of NPIs in the three conditional and conditional-like constructions mentioned above. To solve these problems, a new semantics for each of these constructions is proposed; importantly, I will show that the proposals made in this dissertation not only capture the distribution of NPIs but also account for other syntactic and semantic properties of these constructions. [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.]