Plasmonic nanostructures are extensively used building blocks for engineering optical materials and device architectures. Plasmonic nanocomposites (pNCs) are an emerging class of materials that integrate these nanostructures into hierarchical and often multifunctional systems. These pNCs can be highly customizable by modifying both the plasmonic and matrix components, as well as by controlling the nano- to macroscale morphology of the composite as a whole. Assembly at the nanoscale plays a particularly important role in the design of pNCs that exhibit complex or responsive optical function. Due to their scalability and tunability, pNCs provide a versatile platform for engineering new plasmonic materials and for facile integration into optoelectronic device architectures. This review provides a comprehensive survey of recent achievements in pNC structure, design, fabrication, and optical function, along with some examples of their application in optoelectronics and sensing.