People have used money – and made counterfeits - for thousands of years. Archimedes came up with a clever way of finding out if you’ve been cheated by a goldsmith. Making coins with the right proportions of the right metals presented a huge mathematical challenge for Fibonacci and other mathematicians in the middle ages. This lecture will discuss mathematical elements of coin design, denominations, and why former Gresham Professor of Astronomy Sir Christopher Wren favoured decimal coinage.