The manifold requirements for a world to sustain habitability on long timescales (continuous habitability) are delineated in this chapter. The first part offers a brief introduction to climate physics (e.g., greenhouse effect), and thereupon formulates the notion of the habitable zone, that is, the region where liquid water could exist on rocky planets orbiting stars; the boundaries of the habitable zone as a function of the stellar temperature are also presented. In the second part, the various stellar factors potentially involved in regulating planetary habitability are sketched: winds, flares and space weather, and electromagnetic radiation. The third part chronicles some planetary variables that may affect habitability: mass, plate tectonics, magnetic field, tidal locking, and atmospheric composition. The last part is devoted to examining the high-energy astrophysical processes that might impact habitability on galactic scales: candidates in this regard include supernovae, gamma-ray bursts, and active supermassive black holes.