The role of thermal advection in development is examined in its different guises, from the frontal cyclone model of the Bergen School, through the ideas of Sutcliffe and Petterssen, to the quasi-geostrophic omega equation, system-relative isentropic analysis and the potential vorticity (PV) perspective. The PV formulation of the omega equation introduced by Hoskins et al. (1985) is used, together with a simple quasi-geostrophic numerical model, to compare system relative isentropic upgliding with full vertical velocity. The PV approach is compared with the application of the traditional omega equation, particularly with regard to cancellation between vorticity/potential vorticity and thermal advection terms. An equivalence is proved between the traditional form of the omega equation and the diagnosis of vertical velocity via ageostrophic motion, with the contribution from thermal advection being identified with the vertical motion that arises from isallobaric ageostrophic motion.