Caco-2 cell monolayers mimic intestinal absorptive epithelium and represent a very useful tool for studying transepithelial transport. The literature on Caco-2 cells is controversial regarding transepithelial resistance and permeabilities of different marker compounds across monolayers. This paper discusses probable causes for these discrepancies. First, we present the role of culture conditions, such as the nature of the support or the passage number, on cell biology and transport properties. Further, we compare the presence of transport proteins in Caco-2 cells to mammalian intestinal tissue and discuss their implication for drug absorption. We also examine the advantages and disadvantages of systems such as Transwell and side-by-side diffusion chambers. A summary of comparisons between permeabilities across Caco-2 monolayers and mammalian intestinal tissues is provided. We conclude that the origin of Caco-2 cells and the culture conditions are in part responsible for the discrepancies encountered in the literature.