Ethylene oxide (EO) gas sterilization is one of the most frequently used sterilization methods in medical device manufacture. However, considering that EO is a genotoxic carcinogen, the allowable limits of the EO residue in medical devices after sterilization have been discussed in the international standardization working group, ISO/TC194/WG11, and adopted as an ISO standard, ISO 10993-7 in 1995. This article describes the rationale of these allowable limits and is especially focused on risk assessment procedures.