The concept of dose is fundamental to the discipline of toxicology. For inhaled particles, dose considerations include the sequential processes of inhalation, particle deposition, and particle clearance. Several important parameters modify each of these processes, including environmental, anatomic, and physiologic factors. When such factors are considered, it is possible to identify subpopulations and individuals who are likely to receive particle doses that greatly exceed those for the average population. Higher than average doses can be expected for people who are young, have certain acute or chronic lung diseases, are engaged in exercise, or are exposed in close proximity to sources of air pollutants. Although considerable research has improved the understanding of inhaled particle doses, much is still to be learned before high-risk groups and individuals can be protected properly.