The thesis explores and compares the various sociological and economic theories and relates them to one and other. In the late 19th century Ravenstein formulated his laws of migration. In 1962 Sjaastad formalized the concept of the migration decision as an investment in human capital in his The Costs and Returns to Human Migration. From this perspective followed a first criteria which could be used to model the probability of migration: Expected returns (e.g. in the form of higher wages) need to outweigh costs of migration. Among the latter Sjaastad explicitly included, besides the monetary costs of transport and the opportunity costs associated with the search for a new employment, psychic costs, which he understood as being made up by the migrants preference for a familiar environment and already existing social contacts. By including this non monetary aspect it could henceforth be explained why two regions in the presence of wage differences need not witness any migration between them. In the time since this pioneering work valuable contributions to the field have been made. From a sociological perspective, network theory and the New Economics of Migration are of particular interest and relevance. In the context of migration, the former emphasizes how a network can decrease the costs of resettlement. The New Economics of Migration expands the unit of decision from the individual to the group, thus, assuming that the decision to migrate serves to maximize the utility of a group, for instance a family. The thesis applies the various theories to a series of topics within migration research, such as illegal immigration, the debate over state sovereignty and the brain drain, in order to further the understanding of these phenomena.