The present invention provides a crosslinked foam having an excellent balance of various physical properties in terms of lightness, flexibility, permanent compression set, tear strength, impact resilience, and molding stability; a shoe midsole; and a crosslinkable and foamable composition that provides the crosslinked foam and shoe midsole. A crosslinkable and foamable composition comprising (A) an ethylene-based copolymer; (B) a copolymer comprising a vinyl aromatic monomer unit and an unsaturated bond-containing a conjugated diene monomer unit; (C) an organic peroxide; and (D) a foaming agent; a mass ratio of the component (A) to the component (B), (NB), being from 97/3 to 50/50; and the component (B) comprising 5 mass % or more and 80 mass % or less of the vinyl aromatic monomer unit, and 5 mass % or more and 55 mass % or less of the conjugated diene monomer unit.