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Measurements are fundamental to the advancement of knowledge. In turn, measurements in general need equations that relate measurable physical quantities. The requirement that measured results be widely understandable needs the employment of units, and hence constants. An understanding of the concept of units can thus be of significant help in appreciating the fundamentals of science. With this background, this paper discusses units within the overall context of measurements, and presents the fundamentals of electromagnetic units. The discussion naturally involves a consideration of the electromagnetic constants, namely ε0 (generally called the permittivity of free space), μ0 (generally called the permeability of free space), and c (the velocity of light).