What's going on when we're tetchy, spotty, fearful, tearful or just plain awful? Hormones rule our internal world: they control our growth, our metabolism, weight, water balance, body clocks, fertility, muscle tone, mood, the speed of ageing, whether you want sex or not (and whether you enjoy it) and even who we fall in love with. Their effects may occur in seconds and be over in a flash, or take months and last for thirty years. While we can name some hormones we rarely know what they actually do. But that doesn't stop us claiming that 'it's my hormones' whenever our behaviour seems erratic. As it happens, we're right. It is our hormones. And yes, we are completely in the thrall of things we know nothing about.