Probiotics are live microorganisms that are consumed by a human to treat a variety of pathologies, including inflammatory bowel diseases, constipation, irritable bowel syndrome, diarrhea (acute and antibiotic-related), allergy-related conditions, diabetes, and hypertension. Probiotics contain active live cultures of bacteria, such as lactobacilli, bifidobacteria or lactococci. Probiotics may also contain Streptococcus sp., Leuconostoc sp., Pediococcus sp. and Bacillus sp. Probiotics are living microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Adequate number of living microbes, the presence of specific microorganisms, and their survival in the gastrointestinal (GI) environment are important to achieve desired health benefits of probiotic products. In this in vitro study, 21 leading probiotic formulations commercialized worldwide were evaluated for their microbial content and survivability in simulated GI conditions.