In quantum mechanics, the principle that states you cannot know both the position and momentum of a particle precisely is known as the Heisenberg Uncertainty Principle. Formulated by physicist Werner Heisenberg, this principle highlights fundamental limitations in the measurement of quantum states. It asserts that the more accurately we know a particle's position, the less accurately we can know its momentum, and vice versa. This inherent uncertainty challenges classical mechanics' concepts and plays a vital role in understanding the behavior of subatomic particles, leading to profound implications for fields such as quantum physics and atomic theory.