Constraint definitions in intensity modulated radiation therapy is a key point factor during the treatment planning process. In literature some data are available about dose constraints and volumes according to the tissue architectures. Following ICRU recommendations, organs at risk organized in a parallel structure could receive an acceptably small proportion of high dose component. Mean dose and dose volume histogram is a most convenient tool for incorporating such constraints. Organs described as a serial structure are supposed to receive less than the given maximum dose, directly linked to the occurrence of complications. Dmax is the best way to describe such events. These constraints are new tools in radiation therapy, available for optimizing the dose distribution in target volume, sparing the organs at risk to protect the organ function or at least decreasing the late functional damages like xerostomia. It is necessary to define with accuracy gross target volumes and clinical target volume with available radio-anatomical guidelines before introducing current constraints on each volume in the inverse dosimetry. The management of these constraints remains under the responsibility of the clinicians. A permanent compromise has to be chosen between homogeneity of the dose distribution in the target volume and the probability of preserving functions of organs at risk.