DNA-calcium phosphate coprecipitates have been used for 30 years as an efficient method to introduce genetic material into cells. The method involves simple solutions that can be prepared or purchased by the experimentalist. All the numerous variations of the protocol found in the literature are based on the same principle--a spontaneous precipitation that occurs in supersaturated solutions. When DNA is present during this process, it is readily incorporated into the forming calcium phosphate precipitate. Although a wide range of conditions will lead to precipitates, high transfection efficiencies are only obtained within a narrow range of optimized parameters that assure certain properties of the precipitate. This paper describes several physico-chemical parameters that are critical to adapt the method to a particular cell line and/or cultivation condition. Examples of protocols that were established and tested within the authors' laboratory are presented. The article also emphasizes differences between transfections of adherent and suspended cells.