The need for low density, high stiffness materials has led to the production of magnesium based metal matrix composites using a variety of particles and whiskers as reinforcements. Examples are silicon carbide particles and whiskers, boron carbide particles and chopped alumina fibres. To date most of the techniques used to produce these composite materials have utilised expensive processing technologies such as powder metallurgy, and are only available as wrought products.In order to address the requirements of the automotive industry, Magnesium Elektron has developed a process which combines silicon carbide particles with magnesium alloys in the liquid state, to produce a lower-cost, castable MMC product. This composite has an increased stiffness of 50% higher than conventional magnesium and by controlling the ceramic content of the composite the physical properties such as CTE can also be varied to suit the specific application.The composite can be produced in the form of foundry ingot and can be re-melted using a modification to conventional fluxless magnesium melting technology, provided that simple precautions are taken to prevent contamination and settling-out. Most casting processes will be appropriate to use with this material.This paper will review the development of castable reinforced magnesium alloys at MEL, and will discuss the properties and potential applications of these materials.