Resistance exercise can result in localized damage to muscle tissue. This damage may be observed in sarcolemma, basal lamina, as well as, in the contractile elements and the cytoskeleton. Usually the damage is accompanied by release of enzymes such as creatine kinase (CK) and lactate dehydrogenase, myoglobin and other proteins into the blood. Serum CK has been proposed as one of the best indirect indicators of muscle damage due to its ease of identification and the relatively low cost of assays to quantify it. Thus, CK has been used as an indicator of the training intensity and a diagnostic marker of overtraining. However, some issues complicate CK's use in this manner. There is great interindividual variability in serum CK, which complicates the assignment of reliable reference values for athletes. Furthermore, factors such as training level, muscle groups involved, and gender can influence CK levels to a greater extent than differences in exercise volume completed. This review will detail the process by which resistance exercise induces a rise in circulating CK, illuminate the various factors that affect the CK response to resistance exercise, and discuss the relative usefulness of CK as a marker of training status, in light of these factors.