Wildfire activity is predicted to increase with global climate change, resulting in longer fire seasons and larger areas burned. The emissions from fires are highly variable owing to differences in fuel, burning conditions and other external environmental factors. The smoke that is generated can impact human populations spread over vast geographical areas. Wildfire smoke is a complex mixture of pollutants that can undergo physical and chemical transformation processes during transport and can have major impacts on air quality and public health. This review looks at the main features of smoke that should be considered in the assessment of public health risk. It describes the current state of knowledge and discusses how smoke is produced, what factors affect emissions and smoke distribution, and what constituents of smoke are most likely to cause adverse health effects.