We have long known that good friends can be our main source of moral support, the generous suppliers of love, humour and understanding. But did you know that having good friendships makes you cleverer, richer and healthier? Increasingly, research shows that the single most important factor in a well-lived life is not our jobs, our families, our health, education or wealth – it's the breadth and depth of our friendships. Flora draws on the latest behavioural science to trace the hidden ways in which friendships mould our characters and life chances, and offers fascinating anecdotes and accounts of famous friendships to show us how to make friends, stay friends and, crucially, how to be good friends.