Yeast derivatives including yeast cell wall components are promising alternatives to antibiotics with respect to the promotion of health and performance in livestock, based on their capacity to bind enteropathogenic bacteria and to beneficially modulate the immune system. However, these mode(s) of action both in vitro and in vivo are still not well understood. Furthermore, standardization and reproducibility of in vitro techniques (microbiology, cell culture assays) are critical features for the application of yeast derivatives as well as for the proof of effectiveness. Yeast cell wall products are suggested as anti-adhesive agents and are thus proposed to prevent attachment of certain intestinal bacteria by providing alternative adhesion sites to enterobacteria, which contain mannose-specific type I fimbriae such as Escherichia coli or Salmonella spp. and which is well documented. Various in vitro assay techniques have become of paramount importance for biotechnological research since they allow for determination and quantification of potential mode(s) of action. However, in vitro assays may be criticized by product end users as not accurately reflecting in vivo responses. Pro and cons of different assays and their bias will be discussed specifically regarding yeast cell wall components and adhesion of enteropathogenic bacteria. Immunomodulation is a therapeutic approach intervening in auto-regulating processes of the defense system. Yeast derivatives such as beta-glucans are proposed to interact with cells of the innate immune system by receptor recognition. Controversial data in literature and mode(s) of action are reviewed and discussed here.