The concept of retrofitting became an urgent necessity during World War II, when weapons technology was advancing at an intense pace and planes and ships were becoming outdated even before their construction was complete, and the only solution was to retrofit the completed craft with the brand-new technology. Retrofitting was revived on a massive scale during the energy crisis of the 1970s, when new features were added to millions of old houses to make them more energy-efficient. Retrofitting is thus different from merely renovating, which may not involve any new technology at all.