The paper reviews the source evaluation process with emphasis on the types of information needed from source testing and source modeling. Ideally, the experiments would run long enough to capture all of the emissions from the source. Unfortunately, such testing is not practical for many types of sources such as pressed wood products and many building materials. Then the source emission models and consequent risk assessment must be based on incomplete knowledge of the total source emissions. The types of source data and source data analysis needed for development of source risk assessments are discussed. Suggestions for dealing with imperfectly characterized sources are made. The paper discusses only those aspects of the risk assessment related to the source and source emissions model. Aspects of risk assessment related to activity patterns and health effects of pollutants are not discussed.