Molecules and ions with one or more unpaired electrons have been investigated in matrices, usually the solid rare gases, via electron spin resonance and magneto-infrared spectroscopies. Discussed here are (1) matrix perturbations and quenching mechanisms, (2) structural and wave function information from hyperfine interaction, (3) contributions of spin-spin (dipolar) and the usual dominance of spin-orbit interactions in zero-field splittings, (4) large zero-field splittings requiring measurements in the far-infrared and in high magnetic fields, (5) relativistic effects upon the ordering of low-lying electronic states, (6) application of the superposition model to calculate zero-field splittings from inorganic solid-state data, (7) inner-shell exchange coupling in homonuclear transition-metal and rare-earth-metal diatomics, and (8) solid hydrogen as a matrix.