Magnetospheric substorms represent a global interaction between the solar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is episodically stored in the magnetosphere, with a large fraction of this energy being in the form of excess magnetic flux in the magnetotail lobes. The stored energy is periodically dissipated in an explosive instability that occurs in the near-Earth plasma sheet at substorm expansive phase onset. Methods are discussed to estimate the amount of energy transferred from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the plasma sheet and magnetotail lobes. Possible energy sources to drive substorm evolution are evaluated in one particularly well-observed case (May 3, 1986). By examining the major avenues of energy dissipation, the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, etc. is assessed during the substorm. It is found that stored tail-lobe energy is sufficient (by a factor of 3 or more) to drive observed substorm dissipation processes. On the other hand, energy in the closed field lines of the plasma sheet is insufficient (by a substantial margin) to supply the substorm energy.¿ 1997 American Geophysical Union