Energy-efficient architecture is one of the new topics in our architectural dispute, asked by new legislation but not widely accepted by architects and not always understood by clients. We discuss efficiency of buildings widely, starting with green town-planning and Factor 4 (respectively 10), looking on basics of sustainability and effectiveness, on small houses, re-use of buildings -- we try to define vision of really effective and environment-friendly architecture in intelligible manner. We look in detail on low-energy and passive houses, discussing their principles and showing their best-practice examples. We care with other aspects of efficiency and sustainability too -- alternative building materials, recycling, new technologies and renewable energy sources are discussed too. We suggest key-points of green architecture, expressed in examples of modern low-energy architecture in accordance with environment.