IMPORTANCE OF THE FIELD Biodegradable solid particles are potential carriers for both hydrophobic and hydrophilic drugs and have been marketed for prolongation of pharmaceutical activity. In developing such particles, it is important to achieve stable encapsulation of the drugs in the particles and a controlled rate of drug release. AREAS COVERED IN THIS REVIEW In this paper, the principles and techniques for preparing such particles are reviewed. WHAT THE READER WILL GAIN Overall, it remains difficult to identify a general approach for achieving effective entrapment and controlled release, because these qualities are determined by multiple complex factors. The encapsulation efficiency of drugs in particles can be improved through various techniques, including hydrophobic interaction, covalent bonding, ionic interaction and physical isolation. In addition, the release behaviors of drugs are strongly influenced by environmental conditions as well as the physicochemical properties of the polymers and drugs. TAKE HOME MESSAGE Solid particles with targeting ability in addition to prolongation of biological activity may have potential for development of a new type of pharmaceutical in the clinical field.