The Lebesgue is the standard tool for mathematics, physics, or engineering. It is used where sequences or series arise and where one might wish to express the integral of a sum of integrals or vice versa. It is essential in Fourier analysis and for the construction of Hilbert space and other function spaces. This primer gives a concrete treatment, building Lebesgue theory in a way parallel to the Riemann integral of beginning calculus. It makes the powerful tools of Lebesgue theory readily available to those in applied areas. It is a good introduction for those going on in mathematics as well as a refresher holding new insights for those in the field. Features: * The Riemann-Darboux Integral * The Riemann Integral as a Limit of Sums * Lebesgue Measure on (0.1) * Measurable-Sets - The Caratheodory Characterization