While it is undoubted that clinical evidence supporting the health-promoting activity of probiotic cultures is of paramount importance, it is probably less well appreciated that the technological suitability of these strains is also critical to their exploitation. In this respect, it is not surprising that many human intestinal isolates, many of which are obligatively anaerobic grow very poorly outside their natural habitat, the human gut. Indeed, much of the human intestinal flora are at present unculturable and can only be studied using culture-independent approaches. Consequently, the large-scale cultivation and subsequent storage of probiotic lactobacilli and bifidobacteria in high numbers often presents a major bottleneck to the realization of their commercial potential. For this reason, intensive research efforts have recently focussed on protecting the viability of probiotic cultures both during product manufacture and storage and during gastric transit. These studies have demonstrated that cultures can be significantly protected via encapsulation in a variety of carriers, which include milk proteins and complex (prebiotic) carbohydrates. In many cases, the resultant products not only have better probiotic viability but can also be regarded as 'synbiotics' given the presence of probiotics and prebiotics (Roberfroid 1998). The physiological state of the probiotic cultures being added to a product can also be a major factor affecting overall culture viability. In this respect, the induction of stress responses in probiotic strains can have a dramatic effect on the ability of cultures to survive processing, such as freeze drying and spray drying and during gastric transit. Indeed, we have recently generated probiotic cultures that overexpress the heat shock proteins GroESL and have demonstrated improved performance of the culture under a variety of conditions including heat, spray drying and exposure to gastric acid ( Desmond et al. 2004). The addition of various protective compounds to probiotic cultures can also improve their viability during manufacture - examples include glucose to energize cells on exposure to acid (Corcoran et al. 2005) and cryoprotectents such as inulin to improve survivability during freeze drying (Carvalho et al. 2004). In conclusion, a number of novel technologies are now emerging which can improve the viability of human intestinal strains for probiotic applications, which means that it may be possible to exploit many 'sensitive' cultures which hitherto have been difficult to propagate and maintain at high cell numbers.