Probiotics are defined as live microbial cultures (usually bacterial or yeast-based) that are intended to provide beneficial effects upon consumption. This review will discuss how probiotics are supposed to work, including the functionality of the normal microflora, the types of symbiosis, and the concept of microbial antagonism. Additionally, any potential negative consequences to the consumption of probiotics will be discussed. The question of whether probiotics work better, worse, or equally well on healthy or sick individuals will also be addressed.