The last decade has witnessed growing activities in the field of solid-state magnesium-ion conductors, with magnesium-ion conductivities reaching 10⁻⁴ S cm⁻¹ at room temperature in several material classes. These values are approaching the conductivity of liquid electrolytes used in magnesium-ion batteries, opening up routes towards a future all-solid-state magnesium-ion battery technology, employing a solid-state magnesium-ion conductor as an electrolyte. This chapter reviews the current state of research on solid-state magnesium-ion conductors. It introduces the material classes under investigation, including phosphates, chalcogenides, complex metal hydrides, and metal–organic frameworks and discusses the conduction mechanisms on the basis of their crystal structures and strategies to enhance the ionic conductivity. Challenges faced in their integration into a competitive all-solid-state magnesium battery technology are also addressed.