The principles of gallium arsenide diode lasers are described and the history of developments is sketched. The importance of threshold current density is emphasised and it is shown that confinement of carriers is improved in heterojunctions using gallium-aluminium inter-layers. The separate requirements for confining photons, on the one hand, and carriers on the other, are indicated and it is suggested that threshold current densities as low as 1000 amperes/sq cm are possible in double threshold structures. It is also pointed out that the thinnest active zones, requiring high outgoing photon densities, cannot readily be used because of rapid optical degradation