Colloidal drug delivery systems are one of the subjects investigated intensively in the pharmaceutical field, in recent years. Those systems include microspheres, polymeric nanoparticles, liposomes and fat emulsions. An ideal colloidal system has to carry the active ingredient contained to the site of action and must be able to release it an appropriate rate and time in a controlled pattern. The delivery system must be able to be biologically degradable and degradation products and itself must be nontoxic. There is a contamination risk, with organic solvent residues, toxic monomers and toxic degradation products, of microspheres and polymeric nanoparticles prepared by emulsion polimerization and solvent evaporation methods. Problems of stability, large scale production and targeting of liposomes together with the fast release problem from fat emulsions have lead to solving these problems via searching for an ideal delivery system. For this purpose, in 1991, keeping in mind the combination of the advantages of liposomes, fat emulsions and solid particles and also for the i.v. administration of lipophilic active ingredients, an alternative system of solid lipid nanoparticles (SLN®) have been developed. Many characteristics have been seen to be maintained with these systems where the lipids solid at room temperature were used as carrier matrix materials: low systemic toxicity and cytotoxicity, avoidance from organic solvent residues, large scale production, sterilization using autoclave, controlled release. SLN are prepared by two basic production methods, namely cold homogenization and hot homogenization methods. Generally, cold homogenization method is used for hydrophilic active ingredients while hot homogenization method is used for lipophilic active agents. In the characterization studies of SLN: differential scanning calorimeter and X-ray diffraction apparatus for physical state and localization of active agent; laser diffraction apparatus and photon correlation spectroscopy for particle size, distribution and poly-dispersion index; atomic force microscopy and electron microscopy for determination of the shapes of particles are used. Solid lipid nanoparticular systems show the possibility of sterilization by gamma rays and filtration depending on their formulation, besides autoclaving. SLN has found an application area for the topical, ocular, oral and parenteral administration of active ingredients with its mentioned advantages. The idea of efficient bioavailability for aqueous in-soluble active agents, shows that this delivery system is one of the promising drug delivery systems.