Radio-over-fiber (RoF) systems have recently attracted a lot of attention as a way to integrate wireless and optical systems to enhance communication capacity and keep up with high-speed transmission services. Semiconductor lasers (SLs) play an important role in the development of analog RoF systems due to their high bandwidth. However, when the SL is directly modulated by RF signals simultaneously, the electron and photon densities in the active region are subject to nonlinear transfer characteristics. This generates undesirable distortion products and high-intensity noise in the modulated laser signal, which degrade the signal quality. The review introduces an overview of recent studies on the signal distortions and intensity noise induced by two-tone direct intensity modulation of a semiconductor laser for use in analog RoF systems.