The transition metal ions are optical centres that promise tunable laser operation at room temperature over much of the visible and near-infrared regions. This review presents the essential characteristics used in optical assessment of possible laser-active solids and some of the theory required to understand the experimental results. Also presented is a survey of the main features of the optical absorption-emission cycle, luminescence decay profiles, optical bandshapes and excited-state absorption for Ti3+, Cr3+ and Co2+ ions in (essentially) octahedral environments. Finally, materials characteristics (for example, radiative against non-radiative decay, excited-state absorption and disorder) that limit laser performance are discussed.