Abstract: In World War II non-deforming kinetic energy (KE) projectiles with velocities beneath 1 000 m/s achieved perforation of the target by volume deformation and/or plugging. The design of kinetic energy (KE) projectiles has changed significantly as muzzle velocities have increased, leading to much higher impact velocities. For projectile velocities above 1 000 m/s the mechanisms of volume deformation and plugging are replaced by hydrodynamic penetration, where the projectiles are eroding during the penetration process. This paper provides a brief tutorial on the penetration of kinetic energy (KE) rounds for projectile velocities below and above 1 000 m/s. The corresponding penetration equations are described, after which the paper presents a useful rule-of-thumb, the experimentally obtained diagram of Hohler and Stilp, and an analytical equation by Lanz, Jeanquartier and Odermatt. The paper concludes with a numerical example.