This chapter offers an overview of the attempts that have been made to locate the positions of political actors in policy and/or ideological spaces over the past twenty-five years or so. I claim neither to review all of the relevant literature, nor to explore all of methodologies involved. What I try to do is identify the main strands of research and comment on their limitations and possibilities. The assumption is that the various attempts to define abstract policy and/or ideological spaces and to locate political actors in these are driven by the imperatives of comparative research. In themselves, these approaches offer little to scholars interested only in understanding a single case. Following Sartori’s (1970) usage, the attempt to define a policy and/or ideological space involves moving up the ladder of abstraction, and this makes sense only if the goal is to establish concepts and measures that can travel.