A large number of countries are involved in a process of transformation with regard to the management of municipal solid waste. This process is a consequence of environmental requirements that occasionally materialise in legislation, such as the European Council Directive 31/99/EC on waste release in the European Union. In some cases, the remediation of old landfills can be carried out in compliance with environmental requirements; in other cases, it is necessary to proceed with the closure of the landfill and to assimilate it into its own environment. In both cases, it is necessary to undertake a diagnosis and characterisation of the impacted areas in order to develop an adequate action plan. This study presents a new methodology by which environmental diagnosis of landfill sites may be carried out. The methodology involves the formulation of a series of environmental indeces which provide information concerning the potential environmental problems of the landfills and the particular impact on different environmental elements, as well as information related to location, design and operation. On the basis of these results, it would be possible to draw up action plans for the remediation or closure of the landfill site. By applying the methodology to several landfills in a specific area, it would be possible to prioritize the order of actions required.