In this review, we discuss the evolution of xylem structure in the context of our current understanding of the biophysics of water transport in plants. Water transport in land plants occurs while water is under negative pressure and is thus in a metastable state. Vessels filled with metastable water are prone to dysfunction by cavitation whenever gas-filled voids appear in the vessel lumen. Cavitated vessels fill with air and are incapable of water transport until air bubbles dissolve. We know much more about how cavitations occur and the conditions under which air bubbles (embolisms) dissolve. This gives us an improved understanding of the relations hip between xylem structure and function.