Together with lactic acid bacteria used as the primary starters in cheese or as starters in fermented milks, there is a population of mainly adventitious heterofermentative lactobacilli that may have a positive or negative effect in dairy products. This article describes facultatively and obligately heterofermentative species such as Lactobacillus brevis, Lactobacillus buchneri, Lactobacillus fermentum, Lactobacillus reuteri, Lactobacillus kefiri, Lactobacillus rhamnosus, and Lactobacillus curvatus. General features of these species are described together with desirable (e.g., probiotic activity and flavor formation) and undesirable (e.g., defects from gas production, and flavor and odor defects) properties that affect the characteristics of dairy products.