With 12,000 new cases each year in France, rectal cancers are a frequent entity. Concurrent fluoropyrimidin-based chemoradiation followed by a surgery including total mesorectal excision is the standard of care for locally advanced (T3-4) or node positive cancers of the mid and lower rectum. Modalities of irradiation depend on tumour location (mid versus lower rectum) and its local extension. Nevertheless, the clinical target volume (CTV) always encompasses the entire mesorectum, that goes from the peritoneal reflexion line (facing the third sacral vertebrae) to the levator ani muscles. The internal iliac lymph nodes are as well always included in the CTV. The aim of this article is to review the main epidemiological, anatomical, radiological and prognostic factors that are meaningful to define the optimal modalities of conformal radiation of rectal cancers. Definition of target volumes and organs at risk will be discussed, as well as doses and dose-constraints. A case report will be used to illustrate this article.