One of the main problems of theoretical physics concerns the unification of gravity with quantum theory. This monograph examines unification by means of the appropriate formulation of quantum gauge invariance. Suitable for advanced undergraduates and graduate students of physics, the treatment requires a basic knowledge of quantum mechanics.Opening chapters introduce the free quantum fields and prepare the field for the gauge structure, describing the inductive construction of the time-ordered products by causal perturbation theory. The analysis of causal gauge invariance follows, with considerations of massless and massive spin-1 gauge fields. Succeeding chapters explore the construction of spin-2 gauge theories, concluding with an examination of nongeometric general relativity that offers an innovate approach to gravity and cosmology.