The electrochemical reduction of CO2 over metallic electrocatalysts is a complex process that involves many interrelated phenomena. These phenomena include the composition of both the electrocatalyst and the electrolyte in the immediate vicinity of its surface. Consequently, the properties of both influence the observed activity and product selectivity. This chapter discusses how the composition of the electrocatalyst and electrolyte influence the activity and product selectivity of heterogeneously catalyzed CO2 reduction. The fundamental origins of these influences are discussed in light of the current mechanistic understanding of the reaction and design principles leading to optimal activity and selectivity.