The spectroscopy of trapped ions or laser-cooled atoms offers the prospect of visible frequency standards to match or even exceed the accuracy of the caesium standard. The development of satellite methods for time comparisons has improved by more than an order of magnitude the accuracy with which national laboratories can routinely compare their clocks. Mechanical metrology has not been left behind. Driven by the need to improve manufacturing technology, major advances have taken place in computer control machining and mechanical measuring systems. These, and many other fascinating developments in the field of metrology are presented in this book.