This chapter focuses on two aspects of mesocosm testing: management of test systems and chemical exposure. Although both of these aspects are critical to the conduct of mesocosm studies, little definitive information can be found in the literature which adequately addresses these issues. The Aquatic Effects Dialogue Group spent a considerable amount of time discussing the use of more realistic fish populations and solutions to the fish “problem”. The stochastic nature of both drift and runoff makes accurate predictions close to impossible. In the desire of regulatory authorities to be protective, this uncertainty can lead to an accumulation of worst case assumptions such that mesocosms are often tested under extreme exposure conditions. The effect observed can be influenced by the procedures used for dosing, the trophic status, the water quality of the systems, the life stage of the organisms, and the time of day the systems are dosed.