Bilateral and multilateral development agencies use guarantees in order to reduce investors' exposure to risks and to attract private capital to developing countries. A guarantee is a legally-binding agreement under which the guarantor agrees to pay part or all of the amount due on a loan, or other financial instrument, in the event of non-payment. Across the developing world, there are places where having access to the right guarantee product will enable investments that would otherwise have been blocked-where the returns are there, but the risks involved simply exceed market tolerances, or where regulations limit investors' ability to bear risk. These opportunities are waiting to be seized by bilateral development agencies and development finance institutions (DFIs), who have the flexibility to innovate. Multilateral development banks (MDBs) are the dominant providers of guarantees in certain market segments, where their ability to influence government behavior and to reduce (rather than merely reallocate) risks on the ground gives them a natural advantage. That said, their accounting practices, treatment by regulators, and business models can also constrain them. In other market segments, specialized guarantee providers or DFIs can create tailored guarantees, pricing them in a way that creates a commercially appealing proposition whilst still earning market rates of return on their capital. This report sets out to present the virtues and shortcomings of scaling the use of guarantees, with a special focus on opportunities for innovation by actors that operate outside the established MDB business model. Since guarantees are not a form of financial flow (unless circumstances require calling the guarantee, with the guarantor assuming the debt of the borrower), they differ from other development finance instruments in terms of structuring, costs, and objectives.