Fluid dynamics principles require that circulation of aqueous fluid will be practically ubiquitous in tectonically active parts of the Earth's crust and upper mantle. Both experiment and theory [e.g. Hart, 1971> demonstrate that flow, generally in the form of unicells (Hadley circulation), always occurs for isothermal tilts above a very small critical angle (~5¿), for any non-zero permeability or Rayleigh number, and even for hot over cold geometries. Interestingly, heat transport rates in the unicellular regime are essentially conductive, so such flow, unlike more vigorous flow at higher Rayleigh number, is not properly termed ''convective.'' These principles have numerous geological remifications, including: 1) Many of the hydrothermal systems developed around epizonal intrusions should be dominantly unicellular in nature, which explains their aspect ratios and the smooth and very regular Δ18O variations that are produced in the rocks; 2) Large, long-lived unicells are predicted to occur deep in the Earth's crust wherever Rayleigh numbers are finite and isotherms are substantially inclined, as in zones of batholith intrusion, regional metamorphism, and collision; 3) Unicells with lateral dimensions of several hundred kilometers are predicted to be associated with subduction zones dipping more than 6--12¿, with fluid advection into the hot mantle wedge being instrumental in mantle metasomatism and in the generation of andesitic magmas. ¿ American Geophysical Union 1991