The textbook "Onion Yield Response to Fertilizer Rates Under Irrigation Condition" offers a comprehensive analysis of onion production in semi-arid regions, particularly regarding the effects of fertilizers and irrigation. Based on an experiment conducted in 2007/2008 at Mekelle University in northern Ethiopia, the book examines the effects of nitrogen (N) and phosphorus (P) on the yield and yield components of onions under various irrigation conditions. The study employs a split-plot design to analyze the interactions between different fertilizer rates and irrigation regimes. The results indicate that nitrogen application has significant effects on agronomic data, while the response to phosphorus is irregular. Additionally, the efficiency of water use under deficit irrigation is highlighted. This book is aimed at professionals and students in the field of agronomy and plant production.