In many two-dimensional (2-D) materials, electrons not only possess charge and spin, but further exhibit an unusual quantum feature. Electrons residing in many 2-D materials can live in well-separated energy minima, and the “address” describing which minima these electrons belong to is known as the “valley.” The use of this “valley address” to encode and process information forms the core of a new vibrant research field known as “valleytronics.”