Subtidal rocky communities in the north-west Atlantic are largely limited to latitudes higher than 40°N due to the lack of substrata at lower latitudes. Communities are species poor relative to the north-east Atlantic, and food webs are generally simple, driven by physical processes including low temperatures, water motion and, for more northern regions, sea ice. Whereas kelp should thrive in shallower waters, grazing by the green sea urchin has led to extensive barren grounds. The dynamics vary, however, among regions of the north-west Atlantic, ranging from a kelp-dominated state in the Gulf of Maine to an urchin-dominated state in the Gulf of Saint Lawrence. Cycling between states has occurred in Atlantic Nova Scotia, where urchins are controlled by a disease process unique to this region. Control by predators may have occurred in the past but overfishing has now functionally removed this factor. Certain invertebrate fisheries have developed, and the American lobster appears to be thriving. Outside of the arena of kelp–urchin interactions, diverse assemblages of invertebrates can be found in habitats that range from shallow water bivalve beds to deep-sea coral reefs. Limitations in the temporal and spatial scope of our knowledge severely hamper our ability to generalise.