The talk summarize the empirical state of knowledge on bottleneck flow and introduces an approach to describe crowd disasters. The approach combines quantities well known in natural sciences with concepts of social psychology. It allows to describe crowds at bottlenecks in case of exceptional (life-threatening) or normal circumstances. On the basis of empirical data, the influence of the spatial structure of the boundaries (width and length of the bottleneck) and the motivation of pedestrians on flow and density will be presented. The phenomenon of clogging and its effects on the flow will be discussed in connection with congestion, rewards, motivation and pushing. Positive effects of pillars in front of bottlenecks are critically questioned by recent experiments. Results of two experiments including questionnaire studies connect flow and density with factors of social psychology like rewards, social norms, expectations or fairness.