After a series of observational and theoretical breakthroughs in the 1960s, the Steady State theory was discarded, whereas the Big Bang cosmological paradigm remained viable. This model is described by the Friedmann equations with a Robertson-Walker metric. The metric describes the dynamic spacetime intervals and the Friedmann equations describe the expansion dynamics. The latter are derived from Einstein’s field equations of General Relativity assuming an isotropic and homogeneous medium, conservation of energy density, and an equation of state known as the “continuity equation.” Friedmann’s equations are conveniently written in terms of a time-dependent scale factor, the Hubble constant, and four present-epoch cosmological parameters. Today, we live in an era known as precision cosmology, in which the Hubble constant and cosmological parameters are measured with 1% or better uncertainties. In this chapter, we present an abridged derivation of the Friedmann equations and discuss the cosmological parameters and their temporal evolution in detail. The Robertson-Walker metric is then rewritten in terms of radial and transverse components suitable for convenient practical application.