Many lights and other objects in the sky go unrecognised, or at least are little understood by those observing them. Such things range from the commonplace like rainbows and meteors, to the distinctly unusual like the green flash and ball lightning. And there is still a residuum of objects that remain unidentified by the watcher – classed generally as 'UFOs', a description which today has connotations of the mysterious, even of extraterrestrial visitors.

The first part of this book is an identification guide, very much like the "plant identifier" sections found in a good gardening or botany book. It allows quick (and structured) identification of known aerial phenomena, whether at night or during the day. The objects thus found are referenced to the second part of the book.

The second part gives a full description, physical explanation, and where relevant notes on observing and photographing the various phenomena. Some will need optical aids such as binoculars or telescopes, but the main thrust of the book is identification and explanation rather than imaging.

The final chapter approaches UFOs from a scientific standpoint, particularly the way in which human perception and often preconception affects the outcome. It does however finish with a short section on "extraterrestrial UFOs", emphasising the burden of proof aspect and touching on the scientific theories of life on other worlds and the improbability of visitors.