This paper attempts a concise analysis of the Spanish rules on modification of public contracts. The paper first describes the past situation in which there were no proper limits to modify contracts. The central role of unilateral variations is also stressed. Then it deals with the new 2011 regulation by which national legislation lays down criteria for determining cases in which modifications are allowed. This new regime arises from both the ECJ doctrine and the Commission infringement procedure opened against Spain. The grounds for modifying, the existing limits, and consequences are briefly studied.