The first time you see a map of Paris' Metro system, you might fling it down in confusion and call for a cab. This incredible system was built over many decades. Someone starting from scratch to duplicate it today would run out of money long before finishing. The result can be confounding at times, but when you superimpose a street map on a Metro map, most places worthy of a visit are just a few minutes of walking from a stop. Be careful not to confuse RER trains with Metro trains. They are different systems that sometimes intersect at key stations. You'll need some passes for the Metro (free with a Paris Pass) and some patience as you unravel the routes, but the time and money invested will serve well.