Editorial
In this paper Helmut Elsinger and Martin Summer analyze the pricing of risky income streams in a world with competitive security markets and portfolio constraints. The authors investigate how one can transfer concepts and pricing techniques from a world without frictions to such a more realistic situation. Basically two new aspects arise: The no arbitrage condition has to be replaced by a weaker concept, which is called no unlimited arbitrage. Furthermore an appropriate technique is required for deriving from this concept a pricing theory for contingent claims. The authors show how to achieve this task in a simple way, which is applicable to many relevant constraint situations.