The use of MSS, also known as herbal leys, has increasingly been recognised as a viable alternative in temperate grass-based agricultural systems to enhance forage yield, quality, and ecosystem health. By integrating complementary grass, legume, and herb species into the sward mix, these systems offer enhanced resource-use efficiency, reduced dependency on inorganic N fertilisers, and improved resilience to environmental stressors. Unlike traditional monocultures, MSS leverage the concept of functional diversity, typically utilising four to eight species that interact to maximise productivity. This diversity enables improved nutrient cycling, reduced weed growth, increased soil organic matter, and enhanced carbon sequestration, all of which contribute to ecosystem stability. This chapter outlines the principles, benefits, and practical management strategies of MSS and highlights their importance in creating sustainable and resilient agricultural systems that meet both agronomic and environmental goals.