'An expert romp through the science of extraterrestrial life.' Adam Rutherford Today we know of only a single planet that hosts life: the Earth. But across a Universe of at least 100 billion possibly habitable worlds, surely our planet isn't the only one that is just right for life? As Goldilocks was searching for the perfect bowl of porridge, astrobiologists are searching for conditions throughout the Universe that are just right for life as we currently know it to exist. With the Earth as our guide, the search has begun for similar worlds sitting at the perfect distance from their Sun - within the aptly named 'Goldilocks Zone' - that would enable them to keep water as a liquid on their surface and therefore perhaps support a thriving biosphere. What might life look like on other worlds? It is possible to make best-guesses using facts rooted in biology, physics and chemistry, and by studying 'extremophiles' on Earth, organisms such as the near-indestructible water bears that can survive in the harshest conditions that Earth, and even space, can offer. Goldilocks and the Water Bears is a tale of the origins and evolution of life, and the quest to find it on other planets, on moons, in other galaxies, and throughout the Universe.