In several recent reviews the physiological reasons for the pathological changes seen in birds with avian coccidiosis have been discussed. 1-6 This paper will not repeat information in those reviews unless it is pertinent to new conclusions or overall hypotheses on how coccidiosis induces the observed pathological changes. For example, the effect of anorexia, reduced feed intake, and maldigestion are discussed in several of those reviews but are not covered here because no new information is available. Similarly, many of the references given in those articles will not be given here as it is not the intent of this paper to be a complete review on the subject of pathophysiology. Rather, the emphasis will be on aspects not previously covered or on results obtained.