Abstract: This talk is intended mainly for graduate students. Invariant Theory is the general term for a number of interconnected subjects, including Classical Invariant Theory, Geometric Invariant Theory, and Differential Invariant Theory. All these subjects seek to understand group actions by studying auxiliary functions (invariants or covariants) that transform under the group action in a predictable way. Some examples and results will be presented to illustrate this in the context of Classical Invariant Theory.