Parisi's Theorem is a fundamental result in the field of statistical mechanics and mathematical physics, particularly concerning the behavior of disordered systems. It was proposed by Giorgio Parisi, who received the Nobel Prize in Physics in 2021 for his work. The theorem describes how complex systems, like spin glasses, exhibit a rich structure of states and phase transitions, revealing the underlying organization in seemingly random configurations. The theorem provides a framework for understanding how these systems can be analyzed using concepts from probability theory and functional analysis. It highlights the importance of replica symmetry breaking, which helps explain the emergence of multiple stable states in disordered materials, contributing to our understanding of various physical phenomena.