Circuitry such as electrical components and wires and traces on flexible printed circuits can be embedded within injection-molded plastic structures. The electrical components can include integrated circuits, connectors, and system-in-package circuit modules. The system-in-package circuit modules may include components mounted on a substrate and covered with plastic. The connectors may include a connector for mating with a corresponding connector on an electronic device or a battery. The injection-molded plastic structures may form a housing. The housing may form part of an electronic device, an external case that receives an electronic device, or other structures. A near-field communications antenna may be embedded within a plastic housing. Signal wires and plastic fibers may be interlaced to form a mesh that is embedded in the plastic housing or other injection molded plastic structure.