Probiotics are defined as 'live microorganisms that when administered in adequate amounts confer health benefits on the host', underlining the need of microbial viability and the requirement of a suitable dose to obtain a health benefit. The dose and the administration regimen are critical issues for probiotics either ingested as foods claiming health benefits or used as drugs in clinics. In fact, regulatory authorities demand to guarantee consumers that a probiotic is effective in the recommended conditions of use and responds to its specific claims. Thus, a proper identification of probiotic strain(s), a definition of the amount of microorganisms surviving by the end of the product shelf-life, and a demonstration of their beneficial effects by appropriate human trials are required. The current knowledge on the effective dose of different probiotic strains used for several disorders is here reviewed.