The author tells the 2000-year old history of one of mathematics’ most elusive numbers, the square root of minus one, also known as i, re-creating the baffling mathematical problems that conjured it up and the colourful characters who tried to solve them. Addressing readers with both a general and scholarly interest in mathematics, Nahin weaves into this narrative entertaining historical facts, mathematical discussions, and the application of complex numbers and functions to important problems.