Here, I start with a key paper on searching for broadcasting civilizations written in 1959, which can be regarded as the theoretical beginning of the age of SETI. We move on from there to the first practical SETI endeavour – Project Ozma. I distinguish between incoming and outgoing signals; the latter are sometimes characterized as ‘active SETI’. Regarding incoming signals, nothing has yet been interpreted as having come from alien life. The LGM signal (little green men) of the 1960s was a pulsar; the ‘Wow’ signal of the 1970s remains enigmatic, but no repeats of it have been observed; the FRBs (fast radio bursts, discovered in 2007) are almost certainly not from biological sources. Regarding outgoing signals, we look at the pioneering Morse and Arecibo messages and the various later ones up to the time of writing (2019). So far, no outgoing signal has been replied to, but this is not surprising since most have not reached their targets yet. Next, I turn to the Breakthough Initiatives, launched in 2015. These include an ambitious plan – Breakthrough Starshot – to send micro-spacecraft to the Alpha Centauri system. I end by briefly looking at ethical issues involved in SETI.

Here, I outline the nature of the electromagnetic spectrum. I note that various living organisms can detect ultraviolet, visible, and infrared wavelengths, but that none can detect the longer wavelengths of radio. This may be partly due to the fact that there is little to be gained from evolving such an ability and partly because unrealistically large eyes would be required to ‘see’ these long wavelengths. I then turn to human use of radio, which began in the early twentieth century. I consider the question of when the ‘radio age’ started, from the perspective of our transmissions into space – both accidental and deliberate. Leakage of broadcasts could have occurred since the 1920s; messages specifically aimed into space began in the 1960s. When the radio age will end is hard to predict; guessing how long other broadcasting civilizations will last is even harder. However, using both optimistic and pessimistic figures, I use the Drake equation to guestimate how many broadcasting civilizations there are in the Milky Way right now. The result: anything from just one (us) to about a quarter of a million. I end by stressing the difference between radio waves and radio signals.