We are generally happy to accept that the orbit of a planet can be precisely predicted, or that we can have more and more accurate forecasts of the weather. But we seldom study human behaviour with a similar approach. Every day we take many decisions according to our wish and whim, which makes us think that there is no more unpredictable test subject than a human being. This lecture tells us about a research project which studied the behavioural patterns of millions of anonymous mobile phone users, only to prove this concept wrong. Although our memories are dominated by diversions from our daily routine, measurements indicate that human movements can be predicted with 93% accuracy. The lecture also tells us about the root of this predictability and what its practical implications are – from the estimations of the spread of viruses to understanding mobile phone viruses.