Most existing buildings are affected by various “pathologies” that derive from the methods, materials and building systems originally used in their construction, now totally inadequate in today’s energy conscious climate. As a result they are “energivorous”, consuming up to 20 times the energy necessary to power and heat new buildings. Most existing buildings are in very low energy classes (F and G), and this means high energy bills for the users. The EPDB (Energy Performance of Buildings Directive) 2010/31/EU clarifies, reinforces and widens the scope of the Directive 2002/91/EC. The EPDB recognizes that energy certificates are the first step towards sustainability in existing buildings, providing a first assessment of their efficiency in terms of energy use. It emphasizes how the performance improvement proposals indicated in energy certificates provide an important basis for energy renovations. The activities carried out in an energy assessment bear close relation to those required in an energy audit. Energy assessments are useful in providing a first “diagnosis” of the building, helping us to understand its “state of health” in terms of energy consumption. Energy certificates usually contain brief indications on proposals to improve the buildings energy performance, including retrofits to building systems, thermal insulation of the building envelope, and the use of renewable energy sources. The calculation procedures used for energy certificates are based on standard parameters (external and internal temperatures, hours of operation of heating systems, etc) and so do not tell us the exact amount of energy that a specific building actually uses. Energy certificates are obligatory for the sale and rent of properties, for new constructions or major renovations and retrofits/substitution of building systems, and for access to tax reductions and financial incentives.