Until recently the sources of electromagnetic waves in the radio and $\gamma$-ray bands were much more monochromatic than optical sources. The advent of stable lasers with frequency reproducibility $\sim10^{14}$ has uncovered new possibilities of performing experiments of interest in physics. To illustrate these possibilities, results are cited of the measurements of the quadratic Doppler effect in a gas, the magnetic hyperfine structure of a vibrational-rotational transition of methane, and the recoil effect. A number of experiments that lasers will make possible in the near future are discussed, namely, high-accuracy measurements of the Rydberg constant using two-photon absorption, measurement of the quadratic Doppler shift, measurements of the gravitational frequency shift of the earth's field, and the possibility of observing parity nonconservation in atomic transitions.