The new EU Directives on public procurement demand terminating the contract and issuing a new tender when a material modification is needed. Such a severe consequence is regulated by the Directives using more than desirable uncertain legal terms. This article intends to clarify those legal concepts by examining all cases of substantial and non-substantial modifications and also remedies applicable to the decisions made in this regard by contracting authorities. Furthermore, since 2011 the Spanish legislation is quite similar to the new regulation laid down in the Directives, and a practical approach from the Spanish experience can also be found in this article.