Coding systems are important tools for the documentation of drug-related problems and following interventions. They should be suitable not only for scientific studies but for the broader implementation of Pharmaceutical Care in the pharmacy. A suitable coding system must be easy to use in daily routine. To facilitate later computer aided use, it should be preferably structured like a decision tree and consist of three parts: 1. the classification of drug-related problems 2. the intervention taken to solve to problem 3. the degree to which the problem was solved. PI-Doc considers other classification systems used by Hepler and Strand the PAS-Coding System and draws empirical experience from a study in 1995 and 1998 in Germany. The main groups comprise the following: unsuitable drug choice unsuitable use by the patient, incl. compliance unsuitable dosage drug-drug interactions adverse drug reactions other drug-related problems patient-related communication-related technical and logistic problems. The article discusses basic principles of the development of a coding system and the prerequisites for its application. In addition, it tries to give guidance for the classification process itself. It is also recommended to collect further information while documenting drug-related problems, in order to allow additional statistical evaluation.