From the UV to the IR, we can measure wavelengths as precisely as applications demand -- so far. Spectroscopists have long needed to measure and catalog the "exact" absorption wavelengths of atoms and molecules. But accurate wavelength measurement is not just for spectroscopy anymore. Two applications that require precise wavelength measurements are dense wavelength division multiplexing (WDM) for fiber optic communications and excimer laser lithography for semiconductor chip fabrication. However, their technical challenges are very different. Current wavelength measurement techniques can meet the most extreme requirements of spectroscopy, fiber optic communications systems and deep-UV lithography. The challenge now is system integration.