Uncertainty reflects ignorance associated with population traits (e.g. average exposure levels to a contaminant), with models used to predict risk (e.g. which statistical model is correct), and with a host of other considerations. Variability reflects an intrinsic property of a system (e.g. body mass indices possess a distribution across a population). The incorporation of uncertainty and variability in the assessment of occupational hazards is an important objective. General issues of uncertainty and variability in occupational risk estimation are discussed. This is followed by three illustrations where: firstly, the impact of variability in an exposure assessment and sampling variability in a regression model on risk estimates is considered; secondly, the impact of uncertainty in the size of a workforce on rate modelling is considered; and thirdly, the impact of using different models to predict risk is considered.