Models for precipitation hardening (PH) at room temperature have been available for a long time. In spite of the importance of PH, it took a long time to establish models for elevated temperatures. In fact, empirically the room temperature models have also been used at higher temperatures. This gives the wrong temperature dependence and overestimates PH. It was for a long time thought that it was an energy barrier for climb across particles that was the controlling mechanism, but it was gradually appearing that this effect was so small that it could be neglected. Instead it is time it takes for dislocations to climb across particles that is the critical factor. Small particles are readily passed and do not contribute to the strengthening. Particles larger than a critical size have to be passed by the Orowan mechanism, because there is not time enough for dislocations to climb across these particles. This mechanism was finally verified for Cu–Co alloys.