Book excerpt: Radioactive particles have been released to the environment from a number of sources, including nuclear weapon tests, nuclear accidents and discharges from nuclear installations. Particle characteristics influence the mobility, biological uptake and effects of radionuclides, hence information on these characteristics is essential for assessing environmental impact and risks. This publication presents a series of papers covering sources and source term characterisation, methodologies for characterizing particles, and the impact of particles on the behaviour of radioactive particles in the environment. Sources covered include the Chernobyl accident, nuclear weapons accidents at Thule and Palomares accident, the discharges from Dounreay and Krashnoyarsk, and depleted uranium in Kosovo and Kuwait. The overall aim is that an increased understanding of particle characteristics and behavior will help to reduce some of the uncertainties in environmental impact and risk assessment for particle contaminated areas.