We show that some classes of sterilized interventions have no effect on equilibrium prices or quantities. The proof does not depend on complete markets, infinitely-lived agents, Ricardian equivalence, monetary neutrality, or the law of one price. Moreover, regressions of exchange rates or interest differentials on variables measuring the currency composition of the debt may contain no information, in our theoretical economy, about the effectiveness of such interventions. Another class of interventions requires simultaneous changes in monetary and fiscal policy; their effects depend, generally, on the influence of tax distortions, government spending, and money supplies on economic behavior. We suggest that in applying the portfolio balance approach to the study of intervention, lack 01 explicit modeling of these features is a serious flaw.