In the Administration building at Linkopi ¨ ng University we have one of Oscar Reutersvard’ ¨ s “Impossible Figures” in three dimensions. I call it “Perspectives of Science”. When viewed from a speci c point in space there is order and structure in the 3-dimensional gure. When viewed from other points there is disorder and no structure. If a speci c scienti c paradigm is used, there is order and structure; otherwise there is disorder and no structure. My perspective in Transportation Science has focused on understanding the mathematical structure and the logic underlying the choice probability models in common use. My book with N. F. Stewart on the Gravity model (Erlander and Stewart 1990), was written in this perspective. The present book stems from the same desire to understand underlying assumptions and structure. It investigateshow far a new way of de ning Cost-Minimizing Behavior can take us.Itturnsoutthatall commonlyusedchoiceprobabilitydistributionsoflogittype– log linear probability functions – follow from cost-minimizing behavior de ned in the new way. In addition some new nested models appear.