The final properties of polyolefins are closely related to their distributions of molecular weight (MWD) and chemical composition(GCD). Polyolefins produced with conventional heterogeneous Ziegler-Natta catalysts have broad MWDs and CCDs caused by the presence of several active-site types on these catalysts. These distributions can be adequately described as a weighted superposition of narrower distributions valid for each catalytic site type. With the advent of metallocene catalysts, it is now possible to synthesize polyolefins with narrow MWDs and CCDs, allowing a much better control over the MWDs and CCDs of these polymers. The selective combination of different metallocene types on a given support should permit the production of polyolefins with designed MWDs and CCDs from the knowledge of the MWDs and CCDs of polyolefins made with the individual metallocenes. This paper reviews some recent techniques used to analyze the MWDs and CCDs of polyolefins made with multiple-site-type catalysts and shows how to control the MWD of polyethylene made with a two-site-type silica-supported metallocene catalyst by varying the ethylene pressure during polymerization.