Antenna structures may be customized to compensate for manufacturing variations in electronic device antennas. The antenna structures may include an antenna resonating element and a ground. Customizations may be made to the antenna structures by performing customization operations such as adding material, removing material, deforming material, and making electrical adjustments. Customizations may be performed to a conductive antenna resonating element structure, to a ground structure, or to associated antenna structures such as parasitic antenna elements. During manufacturing operations, antenna structures may be characterized by making radio-frequency antenna performance measurements. Antenna performance can be compared to desired performance levels and compensating customizations for the antenna structures can be identified. Customized antenna structures can be installed in electronic devices during manufacturing to produce devices that meet desired specifications.