A policy paper for CGD. There is a significant and ongoing ramp-up in support for explicitly subsidized official development finance to the private sector around the world, but its role remains poorly defined. Lessons from the aid effectiveness literature as a whole and principles on effective use of aid suggest the need for approaches that do not merely finance the marginal private investment. Regarding experience of government intervention in markets, subsidies are only one of many options to incentivize the private sector, and bespoke subsidies provided by outside actors are rarely likely to be the most efficient form. This paper discusses where outside subsidy of the private sector may make sense and develops principles for the use of aid in subsidies based on that analysis. Subsidies should be allocated on the basis of necessity in meeting public policy goals; the norm for subsidy allocations should be competitive approaches or open offers; non-competitive subsidies should only support market making; subsidy levels should be capped; and subsidy levels should be transparent. Much of the content of these “new” principles is already implied or specified by the existing Multilateral Development Bank Principles to Support Sustainable Private Sector Operations, but they suggest that development finance institutions should not use their standard business model when using subsidies.