Metal additive manufacturing technologies are rapidly becoming an integral part of the advanced technological portfolio for the most demanding industrial applications. These processes are capable of fabricating three-dimensional components with near-net shape quality by depositing the constituent materials in a layer-by-layer fashion. This fabrication approach provides numerous advantages over conventional manufacturing methods, including enhanced design flexibility, reduced production costs and lead times, rapid prototyping, and the possibility to repair damaged parts. In recent years, the growing demand for novel accelerator components with improved performance characteristics, integrating structures such as drift tubes and internal cooling channels, has prompted the exploration of additive manufacturing in the field of particle accelerators. Radio-frequency components, beam intercepting devices, and vacuum systems have been prototyped using various metallic materials and additive manufacturing technologies, demonstrating performance levels comparable to the conventionally manufactured counterparts in preliminary tests. However, the absence of established qualification protocols and the uncertain reliability of additively manufactured parts under the demanding conditions typical of accelerator applications pose significant challenges to the integration of additive manufacturing processes into the fabrication practices of these components. This paper provides a comprehensive review of documented applications of metal additive manufacturing in particle accelerators, highlighting benefits, challenges, and opportunities for future improvements. The main requirements and currently available test setups for the assessment of additively manufactured components in applications involving ultrahigh vacuum and intense electromagnetic fields are also discussed.