<p>Functionally graded materials (FGMs) are materials with a particular characteristic that changes across their various sections. Wire arc additive manufacturing (WAAM) is a highly efficient method of additive manufacturing (AM), and it has been widely used in the manufacturing of FGMs over the last decade. WAAM enables the fabrication of two types of FGMs: discrete and continuous gradients. FGMs can include more than two types of metals or alloys; however, most FGMs produced using WAAM consist of only two different types of metals, forming a structure commonly known as bimetals. In this study we discuss the WAAM system, WAAM modifications to improve FGM construction, various strategies for building FGMs, and characterization of FGMs produced by WAAM. Additionally, various bimetallic parts of FGMs produced by WAAM are reviewed. WAAM has primarily been used to produce Steel-Steel and Ni-Steel FGM parts, with other FGMs involving combinations like Cu or Ti with metals, such as Al, Ni, and Steel. Recent studies have also explored novel FGMs incorporating refractory elements and high-entropy alloys. Furthermore, more complex FGMs, including those incorporating ceramic particles, have recently been developed using WAAM in combination with other manufacturing techniques. The production of each FGM part presents unique challenges that require specific solutions during production. Depending on the type of FGM part, different solutions have been employed, which are discussed in this study. Lastly, the review examines post-heat treatments applied to these materials and their corresponding effects on the fabricated FGMs, along with proposed directions for future research.</p>