The formulation of physical laws depends critically on the system of units chosen to express measurable quantities. This chapter starts with a discussion on the importance of a proper system of units, setting the tone for introducing the natural units in high-energy physics. Particular emphasis is placed on natural units, where the fundamental constants c and \(\hbar \) are set to unity, providing a simplified framework that is especially powerful in high-energy physics, relativity, and quantum field theory. The chapter explores the conceptual and practical advantages of natural units, dimensional analysis, and conversion techniques between different systems of units. Examples from atomic physics, particle physics, and high-energy heavy-ion collisions are provided to illustrate how the choice of units affects the form of equations, the scaling of quantities, and the interpretation of experimental results. The discussion highlights that while the choice of system of units is often a matter of convenience, it is also driven by the scale of physics.