Probiotics are live microorganisms that when administered in adequate amounts confer a health benefit on the host. They are mainly bacteria from the genera Lactobacillus and Bifidobacterium. Traditionally, functional properties of lactobacilli have been studied in more detail than those of bifidobacteria. However, many recent studies have clearly revealed that the bifidobacterial population in the human gut is far more abundant than the population of lactobacilli. Although the 'beneficial gut microbiota' still remains to be elucidated, it is generally believed that the presence of bifidobacteria is associated with a healthy status of the host, and scientific evidence supports the benefits attributed to specific Bifidobacterium strains. To carry out their functional activities, bifidobacteria must be able to survive the gastrointestinal tract transit and persist, at least transiently, in the host. This is achieved using stress response mechanisms and adhesion and colonization factors, as well as by taking advantage of specific energy recruitment pathways. This review summarizes the current knowledge of the mechanisms involved in facilitating the establishment, colonization, and survival of bifidobacteria in the human gut.