The ability to correctly recognize the disease necrotic enteritis (NE) is important not only to those involved in control and treatment of the disease at farm level, but it is also critically important to the search for virulence factors, since a fundamental part of that process is the correct assignation of strains of Clostridium perfringens with respect to virulence. Thus, diagnosticians and investigators need to be able to correctly recognize the lesions of NE. To do this, they must be able to distinguish NE lesions from (1) other enteric diseases such as coccidiosis or viral enteritis, (2) normal features of the intestine, such as the small raised, sometimes red, foci that represent gut-associated lymphoid tissue, (3) autolytic change which may be mistaken for lesions, especially at the microscopical level, by the inexperienced. Errors in diagnosis of NE due to C. perfringens or failure to culture affected areas in which the bacteria of interest with respect to NE are definitively found, might explain some of the early apparently conflicting results with respect to the role of netB in NE. This paper describes at the gross, microscopical and bacteriological level, important features of the intestine of normal poultry and those with NE due to C. perfringens, as well as the common interpretative pitfalls that can lead both to underdiagnosis and overdiagnosis of NE, and to incorrect determination of the virulence of individual C. perfringens strains.