This article presents an overview of 3-D-printed microwave and millimeter-wave components integrating gallium-based (Ga-based) liquid metal (LM). The LM is considered here as a fourth degree of freedom for the design of 3-D-printed antennas, filters, and wireless sensors. The 3-D printing combined with LM technology can be considered as a 4-D printing technology. It can benefit from the advantages offered by 3-D printing technologies (low cost and fast manufacturing) and LM at room temperature (flexibility and reconfigurability). To date, in microwave and millimeter-wave applications, 4-D printing technology is mainly used to achieve the metallization and/or reconfigurability and/or mechanical flexibility of high-frequency 3-D-printed components.