Abstract: This paper summarizes the research on the airfoil trailing edge (TE) tonal noise at low-moderate Reynolds number. Also included are the unsolved problems and their prospected research ideas and roads. The acoustic feedback loop and the vortex shedding are considered to be the two main mechanisms for the generation of airfoil TE tones. The former is formed between the boundary layer and the TE and is associated with the ladder-type discrete tones, while the latter contributes to the single-frequency tonal noise due to global instability. The airfoil TE noise is investigated experimentally in the open/closed wind tunnels and numerically through DNS/LES methods. The development of the T–S waves in the boundary layer is calculated using the linear stability theory, and the vortex shedding is analyzed using the concept of absolute/convective instability. Passive control methods, such as the airfoil leading edge or TE serrations, porous material, and morphing surface, are applied to attenuate the airfoil TE tonal noise. Two main active control strategies are the applications of blowing/suction and the plasma technique. Though the airfoil TE tone at low-moderate Reynolds number has been studied for half a century, some issues still need to be answered, and future development is also prospected.