Metallothionein are small, cysteine-rich, metal-binding proteins that are found ubiquitously in nature. Most metallothioneins bind multiple metals in two well-defined metal-thiolate clusters. This perspective discusses the use of optical spectroscopy to study the metalation of metallothioneins and the emergence of electrospray ionization mass spectrometry as a means of studying the mechanism of metalation for metallothioneins. A brief history of past kinetic studies of cadmium metallothioneins and recent kinetic study advances for the arsenic metalation of metallothionein will be presented. Lastly, a possible functional role for the two-domain structure of metallothionein will be briefly discussed.