An elementary introduction is presented to the study of hadron structure using lattice QCD. Following a brief review of relevant aspects of path integrals, the discrete lattice path integral is presented for gluon and quark fields and used to calculate physical observables. Essential aspects of instanton physics are reviewed, and it is shown how the instanton content is extracted from lattice gluon configurations. Finally, both comparison of results including all gluons with those including only instantons and the study of quark zero modes associated with instantons and their contributions to hadronic observables are used to show the dominant role of gluons in hadron structure. MIT CTP# 2701 hep-lat/9804017 1. – Introduction and Motivation Although a quarter of a century has passed since the experimental discovery of quarks and the formulation of QCD, we are only now beginning to understand the essential physics of the structure of light hadrons. To truly understand hadron structure, one must solve rather than model QCD, and the only known means to do so is the numerical solution of lattice field theory. But obtaining accurate numerical results for observables from a computer is not enough — we also need to obtain physical insight. Hence, our strategy is to use numerical evaluation of the QCD path integral on a lattice to identify the configurations that dominate the action as well as to calculate observables. In recent years, the algorithms and techniques of lattice QCD and the performance of massively parallel computers have developed to the point that we are now on the threshold of reliable, quantitative calculations of QCD observables. Furthermore, there is strong evidence from lattice calculations that the topological excitations of the gluon field corresponding in the semiclassical limit to instantons play a dominant role in the structure of light hadrons. The purpose of these lectures is to describe at an elementary level the basic elements of lattice QCD and how numerical solution of QCD on a lattice is elucidating the role of instantons in light hadrons. To appreciate the significance of the current lattice results, it is useful to recall the wide range of disparate physical pictures that have arisen from the different QCD inspired models introduced to model hadrons. For example, non-relativistic quark models focus on constituent quarks interacting via an adiabatic potential. Bag models postulate a