Perturbation Theory is a mathematical approach used in quantum mechanics and other fields to find an approximate solution to a problem that cannot be solved exactly. It involves starting with a simple system, whose solution is known, and then adding a small disturbance or "perturbation" to it. This allows scientists to calculate how the system behaves under the influence of the perturbation. This technique is particularly useful in situations where the exact solution is complex, such as in the study of atomic and molecular systems. By applying Perturbation Theory, researchers can gain insights into the effects of external forces, like electromagnetic fields, on particles, enhancing our understanding of fundamental physics concepts.