Electron tomography (ET) has emerged as a critical tool for visualizing the three-dimensional ultrastructure of biological specimens at nanometer resolution. This chapter focuses on the application of ET in studying synaptic ultrastructure, in particular postsynaptic density, providing a detailed overview of the techniques and methodologies used to achieve high-resolution three-dimensional reconstruction of synapses. Beginning with an introduction to electron tomography, this chapter delves into the principles of electron tomography, including sample preparation, data collection, and image processing. Special emphasis is placed on the application of cryogenic electron tomography, which allows for the visualization of biological samples in their near-native state. This chapter also reviews the historical context of postsynaptic ultrastructure studies using conventional electron microscopy and explores the significant insights gained using electron tomography, particularly in understanding the nanoscale organization and structural complexity of excitatory synapses. This chapter concludes by discussing the future potential of electron tomography in advancing our knowledge of synaptic biology and its implications for neuroscience.