The aim of phase II clinical trials in oncology is to judge if a new treatment have a sufficient antitumor activity to justify further studies. They represent a crucial step in a new anticancer therapy development. The aim is to present phase II clinical trials planification and interpretation methods, end points, recent methods and news in tumor response assessment. Changes of the place of phase II clinical trials in a new treatment development strategies are finally shown. Non randomized trials planification methods (unique analysis, Gehan’s method, Simon’s procedure, Fleming’s multi-stages procedure, triangular test) are described. Usual primary end point is tumor size diminution. Some studies are interested in secondary end points like survival data or treatment toxicity. New planification methods are exposed : randomized phase II clinical trials, phase II within a phase III trial, cross-over studies and bayesian theories are presented. Recent tumor response evaluation theories (“Recist”) are described. Phase II trials function in new treatment development is progressing : they could be sufficient, in some cases, to allow a temporary marketing authorization if a treatment show great efficacy. Global reflection upon new treatment and methods allowing marketing authorization is required.