Max Planck was a German physicist who is best known for founding quantum theory, which revolutionized our understanding of atomic and subatomic processes. His work laid the groundwork for modern physics, and he introduced the concept of energy quanta, which are now known as photons. Planck's most famous contribution is Planck's Constant, a fundamental feature of quantum mechanics that describes the size of energy quanta. His groundbreaking research earned him the Nobel Prize in Physics in 1918, and he is often regarded as one of the most influential scientists of the 20th century.