Fubini's Theorem states that the double integral of a function f(x,y) can be computed as either \int \int f(x,y) dx \wedge dy or \int \int f(x,y) dx dy, provided the region of integration is classified as "type I" or "type II." If the region does not fit these classifications, it can be subdivided into smaller regions that do. This theorem is essential for evaluating iterated integrals in multivariable calculus. Understanding the types of regions is crucial for applying Fubini's Theorem correctly. Overall, Fubini's Theorem simplifies the computation of double integrals under specific conditions.