This chapter provides an overview of the various types of memory technologies used in modern computing systems. It begins by introducing the memory/storage hierarchy, which encompasses hard disk drives (HDDs), dynamic random-access memory (DRAM), static random-access memory (SRAM), NAND flash, and magnetoresistive random-access memory (MRAM). Each memory type is then discussed in detail, covering its invention, underlying working principles, structural design, and typical applications. The chapter highlights the distinct characteristics and trade-offs among these memory technologies in terms of speed, density, and volatility, offering insights into their roles across different levels of the memory hierarchy. The cover image of this chapter is a TEM result showcasing YMTC’s Xtacking technique in 64-layer NAND in 2018. The periphery circuits and the memory array are wafer-to-wafer bonded. Image courtesy of YMTC.