Probiotics are commonly defined as live microorganisms (yeast or bacteria), when getting ingested in adequate amounts, they exhibit the beneficial effects on the host. During the past two decades, probiotic microorganisms as health-promoting agents have been increasingly added to various types of food products, especially in fermented food and also drugs. Due to the importance of food safety aspects of the human diet and with regards to some adverse effects of probiotics for human, we decided to carry out a review on probiotics and their adverse effects by research in literature. Previous studies indicated that several aspects, including safety, functional and technological characteristics, have to be considered in the selection of probiotic microorganisms. Safety aspects include origin (gastrointestinal tract of healthy human), nonpathogenicity and antibiotic resistance. Some probiotic microorganisms such as enterococci have been considered as an opportunistic pathogen for humans and cause disease, possess agents for antibiotic resistance and potential virulence factors. The bacteria used as a probiotic in food should be completely safe. Probiotic bacteria should be chosen from the healthy human micro-flora and should not have any antibiotic resistance that would prevent treatment of a rare probiotic infection. This review focused on key issues concerning the safety aspects of probiotics added to particular food products for improvement of general health and also discussed the criteria for probiotic selection in details.