The relationship between host and gut microbiota has been the topic of research in recent decades, with particular emphasis on various species of bacteria and their distinct role in health promotion. Early promising research led to the new term probiotic, coined to describe these beneficial bacteria. This early research has laid the foundation for various proposed mechanisms of probiotic effects on health. This article describes current areas of established probiotic use and introduces areas of current investigation, including psychobiotics, which may help patients suffering from psychiatric illness.