A therapeutic trial is generally conducted to determine a new treatment or improve therapeutic applications. Several crucial questions should be considered before the trial is undertaken. The planning of a clinical trial involves a succession of steps intended to develop a protocol providing a clear and unbiased answer to a precise therapeutic question. What treatments are to be compared? What type of population should be studied? What are the evaluation criteria? What is the most suitable experimental scheme? How many patients should be included? What statistical procedures should be implemented (single analysis, intermediary analysis, serial analysis)? This paper considers briefly the main points to be resolved at each planning stage.