Although some classic studies (Hrbacek et al. 1961; Brooks and Dodson 1965) have shown strong effects of planktivorous fish on lower trophic levels, it is only in the last decade that effects of interactions among trophic levels on lake ecosystems have come to the forefront of limnological research (Andersson 1984; Carpenter et al. 1985; McQueen et al. 1986; Northcote 1988; Gulati et al. 1990) A present challenge is to understand the interplay bewtween food web effects and nutrient loading (Benndorf 1988; Persson et al. 1988; Vadas 1989), a challenge that will require comparing experimental manipulations in lakes with varying nutrient loadings (Carpenter and Kitchell 1988; Carpenter et al. 1991). The objective of the food web manipulation in eutrophic Lake Mendota is to establish a large population of piscivorous fish (walleye and northern pike) and, through a cascade of trophic interactions, reduce the planktivorous fish, increase the herbivorous zooplankton, decrease algae, and increase water transparency (Kitchell, Ch. 1). In this chapter we describe the structure of the open-water food web in Lake Mendota and the dramatic changes in the planktivorous fish community that occured during the summer of 1987. The following chapters in this section describe and interpret the behaviour of different trophic levels during the first 3 years of the food web manipulation (1987–89).