The joining together of two previously separate objects. Often used in molecular biology to refer to proteins or small molecules which form links between two or more other molecules - for instance, the protein alpha-actinin forms cross-links between actin filaments in the cytoskeleton. Small cross-linking molecules can be used to join together physically associated proteins in a cell, meaning that those proteins will run as a single band during electrophoresis. This approach is frequently used to identify proteins which associate together as part of a molecular complex.