The invention provides composite materials prepared by i) providing a fluid formulation comprising (1) at least one compound capable of polymerisation and/or cross-linking and (2) a water-consuming reactive filler; ii) optionally injecting said formulation into a site of use; iii) polymerising and/or cross-linking said compound, to form a solid polymer matrix (which may be degradable or non-degradable); iv) causing or allowing said filler to react with water absorbed by said polymer matrix, to produce a solid filler material which is dispersed throughout the composite material. The hydration and formation of the solid filler in situ provides desirable properties to the composites, which have utility for dental composites, bone fillers and adhesives and so on. The composite may also be used to release an active ingredient e.g. an antibacterial or DNA.