Lasers whose output can be varied over a wide range of frequencies offer the opportunity for a new kind of spectroscopy. These tunable sources are welcome in industry, in environmental monitoring and in medicine as well as in basic research. In absorption spectroscopy, for example, tunable coherent sources are bringing about developments, both evolutionary and revolutionary in character, that recall the way powerful fixed‐frequency lasers revitalized the rather moribund field of Raman spectroscopy a few years ago. And an interesting analogy exists between the klystron and microwave spectroscopy on the one hand and tunable lasers and optical spectroscopy on the other.