Bacterial diseases comprise approximately half of the non–outbreak-related mortality in broiler breeders and commercial layers. During the first week of a broiler's life, approximately 50% of the mortality may be caused due to bacterial infections. Outbreaks due to bacterial infections may increase the mortality dramatically and in some cases almost eradicate flocks. E. coli and Gram-positive cocci infections are responsible for mortality and production losses in poultry of all age groups and in all production systems and may be regarded as multifactorial. Subsequently, efforts in understanding and controlling these infections are highly important. Although necrotic enteritis primarily is a disease affecting young chickens, this infection is of major significance in any production system. Besides increased mortality, the production losses observed in subclinical infections may be dramatic. Other classical bacterial infections such as erysipelas and fowl cholera have become more significant in many countries due to increased free-range and organic production. Consequently, this chapter will address these highly important infections.