In the EU a Directive was adopted according to which after 31 December 2020 the construction of buildings that do not meet the requirement of being ‘nearly zero (fossil) energy’ will not be allowed. Today, only a few zero-energy buildings (ZEBs) exist, and they show that their design, construction and operation a dramatic challenge for many reasons. The first is that today’s buildings require too much energy to work, and most of this energy is wasted because of their inappropriate architectural design. This means that architects have to change their design approach and find new ways of obtaining the same (or better) services with much lower energy need. Also mechanical engineers must change their approach, forgetting forever the practice of oversizing plants and systems, with a consequent unnecessary increase in costs and in energy consumption. Still, this is not enough: in order to design a ZEB a new professional must be added to the design team: the energy and comfort expert. This professional (actually a professional’s team, given the complexity of the issue and the sophisticated simulation tools they have to manage) must be integrated right from the earliest phases of the design process. Moreover, the three professionals must work together in an interactive and iterative manner, making the definition of the concept design the most important phase of the entire design process. In other words, for ZEBs the so-called integrated design approach becomes a must. The challenge is not confined to the design phase, but extends to the operation: the best designed building may become an energy wasting one if it is not appropriately operated and if the occupants are not energy conscious. Finally, if we look with a wider perspective, not only Europe with its Directive, but also many other (probably all) countries will adopt the policy of ZEBs, and the number of these buildings will become significant, with a consequent impact on the electric system. In this framework, the issue of mismatch between instantaneous demand and supply of energy will be one of those issues that are crucial in the design, especially at the district scale. The above-mentioned issues are discussed and supported by examples of ZEBs. The design methodology and the technical solutions adopted will be described and the expected performances will be compared with the measured data after more than one year operation.