Coccidiosis is a widespread disease of poultry caused by seven species of Eimeria. It is a significant source of economic losses for the American poultry industry. Subclinical disease, manifested as poor growth and feed conversion, is particularly a problem in commercial broiler and broiler breeder chickens that are 3-5 weeks old. A definitive diagnosis is made via gross lesion and mucosal smear examination done during necropsy, for which live birds are submitted to a diagnostic laboratory. Necropsy findings are distinct for each species of Eimeria, though the pathogenesis of the disease is standard for coccidial pathogens. Anticoccidial drugs, vaccination and management practices are used for coccidiosis control in commercial flocks. Backyard flocks rely on more severe clinical disease signs for a diagnosis. The main differential diagnosis for these signs is necrotic enteritis. Improved vaccination techniques and alternative methods of control are being researched for future use.