Air pollution presents one of the greatest health risks worldwide; hence indoor pollutants have received considerable attention, with a rapidly increasing number of publications in recent decades. This chapter reviews and updates the state of knowledge on indoor aerosols with a focus on their behaviour, physicochemical properties and implications for health assessment studies. It begins with a brief outline of fundamental aerosol dynamics (i.e. deposition, coagulation, evaporation and nucleation) and the main factors that control and affect the concentration and behaviour of aerosols indoors. It then summarizes the concentrations and physicochemical profiles of aerosols in different major indoor sources and microenvironments such as homes, offices and schools. Implications of particle properties for lung dose calculations are discussed. Based on this work, it is concluded that indoor aerosols show a range of particle size distributions and chemical compositions, depending on different indoor emissions and aerosol indoor dynamics. Household aerosols are identified as a main contributor to the total and regional lung dose of ambient particles, especially when expressed by number dose.