Functional particles are prepared directly by heterogeneous polymerization, e.g. emulsion polymerization and dispersion polymerization. Modification of existing particles is another method to prepare functional particles. The features of nano- or microparticles such as large specific surface area, high mobility, easy recovery from the dispersion and reversible dispersibility, etc., are utilized for the exhibition of functions. Medical and biochemical applications of particles, including absorbents, latex diagnostics, affinity bioseparators and drug and enzyme carriers, are the most practical ones at present. Optical and opto-electrical functions of particles are attracting attention. Some particles exhibit unique rheological behavior under special conditions and will expand their applications. In this review article, examples of the above-mentioned functional particles are reviewed and discussed.