Abstract: This presentation will provide a review of the key helicopter rotor noise source mechanisms, their fluid mechanics origins, and an assessment of the state-of-the-art of current noise prediction approaches. Rotorcraft noise is comprised of several components that originate from distinct physical mechanisms, which must generally be treated separately. Furthermore, these distinct noise mechanisms each have unique acoustic radiation patterns that are important to understand the resulting noise of the vehicle. These noise sources are generally well understood and the individual noise sources can be computed with a range of prediction approaches ranging from semi-empirical approaches to first principles methods. Noise prediction is essential for the design of new rotorcraft which will reduce the noise at the source. The challenge in using prediction tools for reducing rotor noise through both design and abatement is twofold: 1) choosing the level of fidelity that captures the important physics with enough accuracy; and 2) reducing both the learning curve to use the tools and the computational power required by the noise prediction system to make the prediction tools accessible to both aircraft designers, operators, and land use planners.