This chapter offers a comprehensive examination of main memory, considering both its architectural aspects and its critical role in systems software. The discussion includes the utilization of physical memory addresses as a linkage mechanism, connecting programs in virtual space to their corresponding execution spaces in the cache and main memory. The chapter also presents advancements in CPU and memory products, elucidating their relevance to memory management. Additionally, it introduces the concept of the OS buffer cache and the development of a key–value store at the user level, highlighting their significance in the broader context of data management systems.