Retrospective dose assessments may be carried out for a number of purposes such as demonstrating compliance with dose limits or investigating possible health implications of past events that led to releases of radioactive materials to the environment. The purpose of the assessment has a major impact on what is assessed and how the assessment is carried out. Any dose assessment requires information about the sources of radioactive material, the way in which they behave in the environment and hence how people will be exposed to the radiation before doses can be estimated. Such doses may also be used to estimate the risks of radiation induced health effects. For a dose assessment to be realistic then there needs to be good understanding of local conditions and the use of any measurements can improve realism. It is important to use the correct dosimetry for the particular assessment which depends on its purpose. It is also important to recognise the uncertainty and variability associated with any dose assessment.