This paper describes the current status and discuss the validity of the fluorescent particle (FP) tracer technique. Properties of the material itself, the blower generator, membrane filter, drum impactor, and Rotorod samplers, and of counting techniques, are described. The inherent and operational errors involved are evaluated, and evidence on the atmospheric diffusion, fallout and impaction, and fluorescent stability of the particles is presents. It is concluded that in the present state of development of the technique the errors, in terms of 90% confidence Intervals, are approximately ±5–10% for source strength determination, ±10–12% (if 300 particles are counted) for dosages determined by the Rotorod, and ±17–20% for dosages determined by the membrane filter sampler. The effects of atypical diffusion on the validity of the method appear to be insignificant, and fluorescence losses may be controlled by proper selection of materials. For ground releases the losses by fallout and impaction may amount to from 1% to 10% during the first few miles of travel, depending on the rate of rise of the cloud and the nature of the ground cover. For larger travel distances, if the cloud height exceeds 100 meters the fallout loss should he below 2% per hour.