Mass accretion onto supermassive black holes plays a key role in galaxy evolution, driving black hole growth and feedback processes that affect the surrounding stars and gas over cosmic time. In this talk, I will discuss recent progress in understanding the central engines of active galactic nuclei (AGN), focusing on the physical properties and structure of the accretion disk and broad-line region. In particular, I will highlight recent results from reverberation mapping and spatially resolved gas kinematics, which provide complementary insights into the structure and dynamics of AGN. Finally, I will discuss future prospects for time-domain and high-angular-resolution studies using space- and ground-based facilities.