Molecular materials exhibit a wide range of electronic, magnetic, and optical properties of fundamental interest and practical importance. What distinguishes these compounds from oxides and semiconductor heterostructures is their significantly lower energy scales, flexibility, and tunable characteristics that enable the development of materials with combined and controllable properties. One overarching theme in this field is the chemical control of physical properties, essentially the observation that the electronic, magnetic, and optical response can be dramatically affected by seemingly minor changes in chemical structure. Physical tunability is a second theme that drives research in the field. This is because the intricate intermolecular interactions that establish crystal and electronic structures also render these materials susceptible to temperature-, magnetic field-, and pressure-induced phase transitions. Light can also be used to control electronic properties and their coupling to structure and magnetism. Combined with the development of multifunctional materials with coexisting or interacting components, structure-property investigations in molecular systems are of contemporary interest and may lead to advances in energy, information, and communication technologies with applications such as spintronics, photovoltaics, nanoelectronics, and solid state lighting. The interplay between charge, structure, and magnetism that controls many phenomena in functional materials can be tested and explored most readily in molecular solids where synthetic chemists, experimental physicists, materials scientists, and theoreticians have traditionally come together to solve interdisciplinary problems.