Important issues that arise in the acquisition, presentation and interpretation of particle size data are discussed. Presentation of size distributions as relative quantity versus size, representation of quantity by number, mass, etc., and procedures for inter-conversion of the different forms are described. Definitions of various kinds of average sizes are presented. Limitations on their use and the importance of precise definition are emphasized. Definitions of size for irregular particles, the role of particle shape and implications with regard to comparability of analytical results based on different principles and procedures are evaluated. Different types of measurement procedures are classified according to whether they involve measurements on individual particles, on separated classes of particles or on complete assemblages. The restrictions and constraints, such as size limitations and resolution which apply to these types, are discussed.