Waves and particles each have unique properties, often ones that are mutually exclusive. But light, classically considered a wave, sometimes behaves like a particle (which we call a photon) and the electron, classically described as a particle, sometimes behaves like a wave. This chapter explores the evidence for wave-particle duality of light and electrons (as well as all other particles), the implications, and possible ways of understanding this duality. In the process, we also introduce and begin to explore the uncertainty principle: the position and the momentum of a particle cannot both be known precisely at the same time.