
The XYZ mesons are unexpected mesons containing a heavy quark and antiquark that have been discovered in the last decade. Their number has grown to about two dozen. Many of them are surprisingly narrow, and several are electrically-charged tetraquark mesons. Their existence presents a serious challenge to our understanding of the spectrum of QCD. I will explain how the Born-Oppenheimer approximation can be used to begin to understand the XYZ mesons.