What were the greatest inventions and discoveries of the 20th century?

I'd argue nuclear weapons were the greatest invention.   It's conceivable that without the Mutually Assured Destruction doctrine  the cold war between the Soviet Union and the USA would have become a hot one at some point.  We can only imagine the death and misery that such a conflict, even contested with conventional means, would have caused.

This!!! Can you make out what I am talking about? Yes, Integrated Circuit. One of the greatest inventions in 20th century. Nothing can beat this. All the technology which we see today was possible only because of this invention.

Difficult as you have: Universal Health Care (Germany / USSR / UK / New Zealand) https://en.wikipedia.org/wiki/Special_relativity (Germany) Airship / Zeppelin (German) Powered, heavier than air, manned flight (USA) Drum brakes (Germany / France) Disc brakes (UK) Diode (UK) Vacuum tube (UK) Electric vacuum cleaners (UK) Bread slicer (USA) Pop-up toaster (USA) Air conditioning (USA) Laminated Glass (France) Speedometer (Croatia) Windscreen wiper (USA) Electric ignition (USA) Radar (Germany / UK) Neon lighting (UK) Cellophane (Switzerland) Teabags (UK) Sonar (UK / Germany / Canada / France) Tank (Australia / UK) Ramjet (France / Hungary) Stainless Steel (UK) Stalinism (USSR) Tape recorder (Germany) Loudspeaker (USA) Insulin (Canada) Television (UK / Hungary / Russia) Fascism (Italy) National Socialism (Germany) Aerosol (Norway) Safety razor (USA) Elektron Microscope (Germany) Cat's Eyes (UK) Jet engine (UK / Germany) Turboprop Engine (Hungary) Atom splitting (New Zealand) Drum Motor (Denmark) Autogiro (Spain) Antibiotics (UK) Philishave (Netherlands) https://en.wikipedia.org/wiki/History_of_the_transistor (Canada / Hungary / Germany) Computer (UK / Germany) Synthetic rubber (Germany) HUD / Head Up Display (UK) Cruise Missile (V1 = Germany) Ballistic missile (V2 = Germany) Assault rifle (Germany / USSR) DNA (UK / USA) Lego (Denmark) Pacemaker (Australia / Canada) Rotary Clothes Line / Hills Hoist (Australia) Ejector seat (UK) Jigsaw power tool (Switzerland) Polaroid camera (USA) Maoism (China) Hovercraft (New Zealand / UK) Satellite (USSR) Velcro (Switzerland) Scotch tape (USA) Electric Dishwasher (UK) Automatic Kettle (UK) Microwave Oven (USA) Baby Buggy (UK) Float Glass (UK) Carbon fibre (UK) Three-point safety belt (Sweden) Rollerball / Gel pen (Japan) Quartz watch (Japan) Instant noodle (Japan) Food Processor (France) Laser (USA) ATM / Cash machine (UK) Pocket calculator (Japan) Orbital Engine (Australia) Photocopier (USA) Heart transplant (South Africa) Internet (USA) Walkman (Brazil / Japan) CAT Scan (South Africa) Compact cassette (Belgium / Netherlands) Power strip (Australia) Rubix Cube (Hungary) Tetris (USSR) VHS / Betamax Video recorders (Japan) Computer virus (Pakistan) Compact Disc / DVD (Japan / Netherlands) ARM architecture CPU's (UK) Web (CERN / UK) Wind-up radio / torch (UK) Speed Gun (South Africa) Bluetooth (Sweden) Tanks with tea brewing facilities (UK) See also: https://en.wikipedia.org/wiki/Timeline_of_historic_inventions

The Washing machine is definitely one of them. As explained by .

I would have to say microchips.  The internet is huge but wouldn't exist in its current form without the invention of microchips.  Of course microchips wouldn't exist without the invention of the transistor.

Gas turbine The diesel powered ship, truck, train Nuclear fission as both a power source and a weapon GPS

Anything that makes the world a smaller place and anything that helps us to be more self sufficient. Commercial air travel Solar energy Modern hydroponics Water purification systems Nuclear power The internet Satellite communication Sealed double glazing

There's been a great amount of discussion on this topic but academic papers usually point out to the car, the telephone and the computer. Because they have ignited other inventions (like the Internet followed the telephone etc).

Probably the invention of the transistor, which enabled the birth of the Age of Information. Whether this was a good or a bad thing will be decided by posterity.