Horizontal evaluation combines self-assessment and external evaluation by peers. Papa Andina, a regional network that works to reduce rural poverty in the Andean region by fostering innovation in potato production and marketing, has used horizontal evaluations to improve the work of local project teams and to share knowledge within the network. In a horizontal evaluation workshop, a project team and peers from other organizations independently assess the strengths and weaknesses of a research and development (R&D) approach being developed and then compare the assessments. Project team members formulate recommendations for improving the R&D approach, and peers consider ways to apply it back home. Practical results of horizontal evaluation have included strengthening the R&D approaches being developed, experimenting with their use at new sites, improvements in other areas of work, and strengthened interpersonal relations among network members. © 2007 American Evaluation Association.