Purpose: Maize output is decreasing in developing countries due to poor postharvest management despite the huge investments. The study is aimed at identifying and quantifying sources and causes of postharvest losses from farm to retail and suggest appropriate interventions for reducing these losses. Research Method: Commodity system assessment on postharvest handling, storage and marketing of maize was conducted between July and August, 2017 in Nigeria, Rwanda and Punjab (India) using surveys, interviews, observations, measurements and quantification of losses along the value chain. Postharvest quality and economic loss assessments along the chain were based on physical damage, decay and weight loss. Findings: Causes of postharvest losses at farm level are similar in all the countries studied, with sorting losses from 2 to 50% and drying losses from 10 to 40%. Damage and defects were extremely high in Rwanda at the farm level. Maize is stored for 5 to 10 months with 2 to 3.5% damage in Nigeria and 6% in Rwanda. Maize is stacked in sacks during wholesale and cleaning rejects obtained is 2 to 11% in Nigeria and India, little or no sorting in Rwanda at wholesale. Postharvest losses are 15%, 60% and 20% of total produce for Nigeria, Rwanda and Punjab, which account for economic losses of $720 million, $131.2 million and $8.2 million respectively. Limitations: The research is limited to major production areas in the three countries because of funding. Originality/Value: The study identified problems, sources and causes of post-harvest losses and suggests appropriate interventions, training needs and advocacy issues to reduce these losses.