Pulse arrival times of pulsars have a high – though limited – degree of predictability: they show both discrete ‘glitches’, and continuous deviations from an analytic extrapolation of earlier data spans; they are ‘noisy’. This noise shows a strong dependence on a pulsar’s spindown age. Both types of noise can be understood as due to a loose coupling of the charged component of a pulsar’s moment of inertia to a neutral superfluid component, of relative moment of inertia usually below 1%, but reaching 10% in extreme cases. Differential spin rates δΩ/Ω of the two components are ≲ 10−3, usually only ≲ 10−5.