Departures from equilibrium are, therefore, extremely important in determining the abundance of the relics that can be observed today. In this chapter, the authors apply the considerations to neutrinos, in order to see what can be inferred about their masses. They attempt a similar analysis for ‘axions’, hypothetical particles that are required to exist if the ‘strong CP problem’ of the standard SU(3) × SU(2) × U(1) model of particle physics is solved by the ‘Peccei–Quinn’ mechanism, currently the only known solution of this problem. Axions must be very light, like the neutrinos. If they have survived until the present, their mass too is strongly constrained by various astrophysical and cosmological data. In any event, it is clear that thermal axions cannot provide anything like the measured matter density.