This chapter discusses some of the reasons why fine particles are characterized in terms of size, composition, or concentration. It also discusses the problems in handling particles before and during the characterization processes, including some of the procedures in common use for sampling and the physical phenomena which affect materials in the sample. The chapter describes the methods and instruments for characterizing fine particles in terms of their operating principles and procedures, particularly for commercially available devices. It examines both the capabilities and limitations of these instruments. The chapter provides some descriptions of necessary ancillary elements for practical instrument operation, ranging from sample feeding devices to computer interface devices. It also describes some typical applications along with problems and their solutions as have been encountered in the particle size measurement area. Particle concentration measurement is primarily of concern in defining the content of materials in liquid suspensions.