The active laser medium (also called a gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source.

Examples of active laser media include:

In order to fire a laser, the active gain medium must be changed into a state in which population inversion occurs. The preparation of this state requires an external energy source and is known as laser pumping. Pumping may be achieved with electrical currents (e.g. semiconductors, or gases via high-voltage discharges) or with light, generated by discharge lamps or by other lasers (semiconductor lasers). More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams.

The simplest model of optical gain in real systems includes just two, energetically well separated, groups of sub-levels. Within each sub-level group, fast transitions ensure that thermal equilibrium is reached quickly. Stimulated emissions between upper and lower groups, essential for gain, require the upper levels to be more populated than the corresponding lower ones. This situation is called population-inversion. It is more readily achieved if unstimulated transition rates between the two groups are slow, i.e. the upper levels are metastable. Population inversions are more easily produced when only the lowest sublevels are occupied, requiring either low temperatures or well energetically split groups.

In the case of amplification of optical signals, the lasing frequency is called signal frequency. If the externally provided energy required for the signal's amplification is optical, it would necessarily be at the same or higher pump frequency.

The simple medium can be characterized with effective cross-sections of absorption and emission at frequencies ${\displaystyle ~\omega _{\rm {p}}~}$ and ${\displaystyle ~\omega _{\rm {s}}}$.

The relative concentrations can be defined as ${\displaystyle ~n_{1}=N_{1}/N~}$ and ${\displaystyle ~n_{2}=N_{2}/N}$.

The rate of transitions of an active center from the ground state to the excited state can be expressed like this: ${\displaystyle ~W_{\rm {u}}={\frac {I_{\rm {p}}\sigma _{\rm {ap}}}{\hbar \omega _{\rm {p}}}}+{\frac {I_{\rm {s}}\sigma _{\rm {as}}}{\hbar \omega _{\rm {s}}}}~}$.