Milk lipids provide various nutrition and health benefits to the consumer. However, these components are not secreted by the mammary gland epithelial cells as isolated components. They are secreted as part of complex molecules such as triglycerides and polar lipids like glycerophospholipids and sphingolipids, which are assembled into a macrostructure termed the milk fat globule. There is now compelling evidence of the bioactive roles exerted by these polar lipids. In this chapter, the bioactivity of milk lipids is discussed with a special focus on the complex structure utilized by the mammary gland to transport lipids into milk. The chapter provides a detailed description of the synthesis and secretion pathways of lipids and the metabolic mechanisms underlying these secretory pathways. The chapter also describes how metabolic pathways modulate the composition and bioactivity of milk lipids.