Atomic magnetometers are used for sensing very small magnetic fields (down to ∼1 femto Tesla), where applications include biological imaging, geology, global positioning and fundamental experiments. An all-optical atomic magnetometer operates as follows: 1 i) A vapour of alkali atoms (K, Rb or Cs) within a glass vapour cell have randomly orientated atomic spins; ii) Circularly polarised pump light aligns the spins with the pump beam and when a small magnetic field is applied the spins precess in the magnetic field; iii) The orientated spins rotate the polarization of linearly polarised probe light (Faraday rotation); and iv) The intensity or polarization of the transmitted probe light measures the magnetic field strength. An anti-relaxation coat on the cell walls preserves the spin polarization (lifetime) of the alkali atoms. A key feature is the importance of designing cell wall coatings with good relaxation properties for these experiments. For a comprehensive discussion of alkali-metal vapour cells and anti-relaxtion coatings, including surface fabrication and characterisation techniques, the reader is referred to the review of Chi et al. Moreover, Wu has reviewed the progress that has been made, over the past six decades, in understanding the nature of the wall interactions of spin-polarized atoms. In a spin-exchange relaxation-free (SERF) magnetometer pumped alkali atoms are in an excited state with hyperfine level F ′ . As an example, the D 1 line (5 2 S 1/2 to 5 2 P 1/2 transition) in Rb was used to directly measure the average dwell time of spin polarized rubidium atoms on coated glass surfaces. The F = 1 to F ′ = 1, 2 transition (optical pump) creates spin polarisation in the Zeeman multiplets (M F ) of the ground state F = 2 hyperfine level. Probe light tuned to the F = 2 to F ′ = 1, 2 transition (D 1 line) probes the population in the F = 2 state. The spin polarisation is monitored by the rotation of the polarisation plane (Faraday rotation) of the probe beam. When two spin polarised atoms collide the electrons can transition into the other hyperfine state, or precess (change in orientation of rotational axis) in the opposite direction from the bulk of the ensemble, thereby causing decoherence and loss of signal. Spin-exchange relaxation is suppressed if collisions occur fast enough in low magnetic fields. In such a regime the spins do not have enough time to precess and decohere between collisions. This is achieved by operating with sufficiently high alkali metal density (at higher temperature) and in sufficiently low magnetic fields.