Remarkably narrow optical homogeneous linewidths of the order of 1 kHz have now been observed in low temperature zero-phonon transitions of dilute impurity ion crystals, such as Pr3+ in LaF3. Novel nonlinear optical resonance techniques have been devised for this purpose using ultrastable phase locked cw dye lasers where the measurements are performed either in the frequency domain (hole burning) or in the time domain (coherent optical transients). These studies effectively bring the Mossbauer effect into the optical region. Hence, the observed linewidths are no longer limited by inhomogeneous strain broadening (~5 GHz) or even by static local fields due to neighboring spins (~100 kHz). However, weak magnetic field fluctuations from local spins are readily detected. As an example, spin decoupling and line narrowing, which are well known in NMR, are observed in an optical transition of Pr3+:LaF3 at 2°K where the 19F–19F dipolar interaction is quenched and the optical line width drops from 10 to 2 kHz, clearly demonstrating the spin broadening mechanism. Results will be discussed in terms of a Monte Carlo line broadening theory.