Supercritical fluid processes are of increasing interest for many fields: in supercritical fluid separation (petroleum-chemistry separation and purification, food industry) and supercritical fluid chromatography (analytical and preparative separation, determination of physicochemical properties); as reaction media with continuously adjustable properties from gas to liquid (low-density polyethylene, waste destruction, polymer recycling); in geology and mineralogy (volcanoes, geothermal energy, hydrothermal synthesis); in particle, fibber and substrate formations (pharmaceuticals, explosives, coatings); in drying materials (gels). This paper presents the unusual physicochemical properties of supercritical fluids in relation to their engineering applications. After a short report of fundamental concepts of critical behavior in pure fluids. we develop in more details the tunable physicochemical properties of fluid in the supercritical domain. The second part of this paper describes the engineering applications of supercritical fluids relevant of chemical reactions and polymer processing. Each application presentation is divided in two parts: the first one recalls the basic concepts including general background, physicochemical properties and the second one develops the engineering applications relevant of the advocated domain.