The basic polarisation properties of spiral antennas are considered by means of circularly polarised spherical waves (CPSW). At first, specific properties of the CPSW and their impact on the antenna polarisation are discussed and conditions for the radiation of pure circular polarisation are identified. Then, symmetrical multi-arm structures are introduced as a general class of antennas which permits an optimisation of the polarisation performance by a suitable control of the CPSW spectra. Finally, the specific CPSW spectra of planar spirals - one member of this class - are discussed. An explanation of the polarisation properties of planar spirals and some design guidelines are derived from this discussion.