Spheres lodging in the microcirculation have proven to be very useful for measuring regional organ blood flow. While radioactive microspheres were most popular, their use is diminishing due to increasing restrictive legislation and costs of storage and disposal as well as the desire to reduce radiation load in general. Fluorescent microspheres (FM) were not the first, but until now the best non-radioactive alternative. Advantages over other types of non-radioactive spheres are higher sensitivity and better spectral separation. FM even compare favorably with radioactive microspheres with respect to spectral separation, life time of dyes, release of dyes from spheres over time and the possibility to separate different colors of spheres in histological preparations. A major disadvantage is, however, the more time consuming sample processing with possibility to lose spheres. This presentation will review the possible sources of errors of each of the microsphere methods, particularly the FM method. Also attention will be payed to various ways of sample processing and quantification of the number of spheres. The FM method has been validated for almost all organs and under various conditions. A basic set of validations to be used by each investigator will also be discussed.