The interchange instability is reviewed with particular emphasis on its relevance to planetary magnetospheric processes. Results of earlier work are reviewed and drawn together by the derivation of a general stability condition, based on a small perturbation approach, for a plasma with isotropic pressure contained in a curved magnetic field in the presence of gravity. The results are not restricted to low pressure (low β) or special field geometry, and the straight field result is recovered in the appropriate limit. We use a small perturbation approach for the instability analysis. Conditions under which an effective gravity may be introduced to simulate rotational effects are discussed. Recent criticism of the Jovian ring current impoundment hypothesis which was based on the straight field line results is shown to be ill founded.