The major challenges in clinical oncological drug studies are to detect promising drug candidates early on in clinical drug development and to protect patients from adverse drug reactions of ineffective drugs. These challenges are met in the classical concept of clinical oncological drug trials in three phases. The aim of the present article is to provide a concise and comprehensive overview of the scientific logic of these 3 phases. Recent progress in the conception of clinical oncological drug studies was made in particular in phase I where the implementation of novel dose escalation procedures has led to a speeding up of phase I and an increase in the number of patients that receive an experimental drug in the therapeutic concentration range. However, the classical 3-phase concept of clinical oncological drug trials can not be transposed to trials with some novel anticancer drugs without critical reappraisal. It is therefore expected that this classical concept will undergo specific modifications in the near future. Copyright 2000 S. Karger GmbH, Freiburg