The Northwest Energy Efficiency Alliance (NEEA) contracted with Evergreen Economics to conduct a market characterization of the commercial market for secondary windows in the Northwest. This work was conducted between August 2019 and January 2020. Secondary windows are retrofit products which use a frame to add one or more additional layers of glazing to an existing window for improved energy efficiency. The findings of this qualitative market characterization address several objectives, such as documenting the supply chain; understanding market actors’ perceptions of secondary windows and their perceived possible and best applications in commercial buildings; and identifying barriers and opportunities for secondary windows in the Northwest commercial market. The report contains recommendations for market and field tests to further NEEA’s efforts to transform the commercial retrofit market for more efficient windows.