Episodic calc-alkaline igneous activity in magmatic arcs contrasts with steady-state magmatism accompanying seafloor spreading. Gilluly1 examined this apparent anomaly in western North America and he and Lanphere and Reed2 concluded that igneous activity has been more or less continuous over the entire North American Cordillera since the Triassic. A similar view was held by Pitcher3 for the coastal batholith of Peru. Numerous studies, however, support the view that episodic magmatism characterizes areas of local to regional scale along convergent plate margins1,2,4–12. Grow and Atwater13 suggested that increased magmatic activity in Alaska might have been induced by subduction of a spreading ridge. If so, episodic igneous activity could be produced by successive subduction events involving spreading centres8. Dickinson7 suggested a model involving frictional heating along the Benioff zone, accumulation of magma above that zone, and periodic magmatism triggered by an unknown condition or event. However, he later8 proposed that regional episodicity is a function of the interaction between a specific site and a wavering (migrating back and forth) magmatic axis that occasionally passes through the site to produce a magmatic event. Migration of the magmatic axis results from accretion or erosion at the trench, variation in the dip of the subduction zone, and (or) reorganization of plate movements8,14–16. Shaw et al.17 suggest tidal power as a source of mechanical energy which, through periodic variations in shear melting of the mantle, produces magmatic periodicity. In contrast, Gilluly1 suggests that variations in the properties of the mantle and of the crust of the subducting plate may be responsible for magmatic episodicity. We propose an alternate explanation for local to regional plutonic episodicity. Although this ‘episodic accretion plutonism model’, is based on data from California, it is applicable to other segments of orogenic belts including southwestern Alaska, southern Japan and western Indonesia. We limit the discussion of igneous activity to plutonism, as geochronological data suggest that volcanism is not simply the surficial expression of plutonism. This possibility is supported by a recent re-evaluation of Andean magmatism18. We shall discuss the problem of volcanism and its relationship to accretion and plutonism elsewhere.