The present invention relates to a method of making a composite sandwich structure. The method comprises the steps of: providing a base lay-up on a surface, wherein the base lay-up comprises a first reinforcement material layer; providing a core layer on to the base lay-up, wherein the core layer comprises an open cellular structure and wherein the open cellular structure is at least partially filled with unbound particles; providing a top lay-up on top of the core layer, wherein the top lay-up comprises a second reinforcement material layer; sealing the arrangement within a hermetically sealed enclosure; introducing matrix material into the arrangement via the at least one conduit through a pressure differential; and curing the matrix material.