The single-site nature of metallocene catalysts permits the production of polyolefins with narrow distributions of molecular weight, chemical composition and comonomer sequence length. This enhanced control over polymer chain microstructure permits, in principle, the synthesis of polyolefins with well-controlled properties. In order to benefit from this enhanced control, one needs to understand how polymerization conditions and catalyst type affect polymer microstructure, as well as how the existing characterization techniques can be used to determine their microstructure. This manuscript investigates the effect of polymerization conditions and catalyst type on the chemical composition distribution (CCD) of ethylene/alpha-olefin copolymers made with single-site and Ziegler-Natta catalysts. Mathematical models will be used to describe CCD of polyolefins made on single- and multiple-site-type catalysts. Crystallization analysis fractionation (CRYSTAF) and temperature rising elution fractionation (TREF) are used to measure the CCD of these resins. It will be shown that the interpretation of CCDs with these mathematical models can be used to enhance our understanding on the nature of active sites present in metallocene and Ziegler-Natta catalysts.