Apartment buildings are home to more than 17 million households nationwide, yet they remain a significant and mostly untapped opportunity for energy efficiency gains. Energy efficiency upgrades in multifamily buildings could save building owners and residents up to $3.4 billion annually. Many cities and states that have embraced energy retrofitting as a job creator and boon to both the environment and economy have yet to address potential savings in multifamily properties, primarily because of obstacles not faced by single-family and commercial properties. The need for multifamily energy retrofits is clear, but two barriers—a lack of information and financing—stand in the way. This paper reviews the challenges facing the multifamily housing energy efficiency market and the opportunity that tools such as on-bill repayment provide to finance energy efficiency retrofits for the subsidized affordable multifamily market.