Radar began to be widely adopted by air traffic controllers in the 1950s and is still the mainstay of most air traffic control systems around the world today. BUT future technology and high skills and knowledge can do above normal practice. Thats why Missing of MH370 cannot be detected

http://www.cbc.ca/news/world/how-could-malaysia-airlines-flight-mh370-go-off-the-radar-1.2568713

A map shows flights tracked by Flightradar24.com, which has 3,200 receivers deployed around the world that track planes using a technology known as ADS-B. Two of its receivers in Malaysia picked up Malaysia Airlines Flight MH370 over the Gulf of Thailand up until the point its responders stopped emitting the radio waves that help track its movements. (Courtesy of Flightradar24.com)

A diagram shows how the ADS-B system of tracking planes uses signals sent from GPS satellites and plane transponders to relay information such as a plane's location, speed and flight number to radio receivers on the ground. (Courtesy of Flightradar24.com