Flame retardant compositions are disclosed which comprise (a) at least one particulate material which expands on the application of heat and (b) at least one particulate nano-filler, together with at least one polymer and/or at least one curable monomer or oligomer. The compositions may also contain certain silicon-based materials. Flame-retardant compositions comprising polyorganosiloxanes containing one or more functional groups selected form amino, hydroxyl, methacrylic, acrylic and epoxy groups, are also disclosed.