Optical techniques for particulate analysis have inherent and significant advantages over those conventional methods which involve batch sampling and subsequent analysis. Optical particle diagnostic techniques can be divided into three broad categories: ensemble methods which analyze the integrated optical properties of an assembly of particles; single particle counters (SPC) which analyze individual particles; and, finally, imaging techniques based on photography or holography. This paper is primarily concerned with laser-based single particle counters applicable to in situ sizing of particles and droplets in the diameter range 0.1 um to 1 mm. Theoretical principles of the various single particle counter designs are discussed with emphasis on comparing performance characteristics and relative advantages. The effects of size-dependent sample space, nonspherical particles, unknown particle composition, and applicable size ranges are considered.