As global food demand increases, sustainable alternatives to synthetic fertilizers are urgently needed. Frass, a by-product of insect farming composed of insect excreta, exuviae, and uneaten feed, is gaining attention as a nutrient-rich organic amendment. This chapter explores the agronomic potential of frass, highlighting its composition, nutrient availability, and impact on soil health and plant productivity. Frass supplies essential macronutrients (N, P, K), secondary elements, and beneficial organic compounds like chitin, enhancing soil biological activity and plant health. However, its efficacy varies depending on insect species, rearing substrate, and application rate. While frass aligns with circular economy principles by recycling organic waste into valuable agricultural inputs, challenges such as nutrient imbalances, salinization, long-term effects and the need for standardization must be addressed to optimize its application and ensure its safe and effective integration into sustainable farming systems.