Explore the physics of electron-ion collisions in this lecture from the International School and Workshop on Probing Hadron Structure at the Electron-Ion Collider. Delve into the fundamental aspects of e-A collisions, their significance in understanding hadron structure, and their role in the upcoming Electron-Ion Collider (EIC) experiments. Gain insights into how these collisions will help probe the internal structure of protons and neutrons with unprecedented detail, investigating key questions about quark and gluon distributions, nucleon mass composition, and the potential for gluon saturation in heavy ion collisions. Learn about the wide kinematical coverage and high luminosity of the EIC, and how it will provide access to previously unseen information about nucleon structure.