Intact eggs can become contaminated with Salmonella as a result of infections of the reproductive tissues of the laying hens but also by penetration through the shell. In this paper, the penetration of Salmonella through the shell of hen eggs is reviewed. A description is given of the advantages and disadvantages of the various methods used to study bacterial penetration through the shell and membranes. The possibility of Salmonella contamination of the shell after lay is included because shell contamination is the first requisite for penetration. Various factors affect the probability of bacterial penetration. Both the intrinsic and extrinsic factors are highlighted. For the extrinsic factors, the influence of bacterial strain and number of organisms, temperature, moisture and immersion and storage conditions on the probability of Salmonella penetration through the shell is described. With regard to intrinsic factors, the presence of cuticle, shell characteristics (shell quality, porosity, shell defects) and membrane properties are summarized.