Monolithic 3D integration technology has emerged as an alternative candidate to conventional transistor scaling. Unlike conventional processes where multiple metal layers are fabricated above a single transistor layer, monolithic 3D technology enables multiple transistor layers above a single substrate. By providing vertical interconnects with physical dimensions similar to conventional metal vias, monolithic 3D technology enables unprecedented integration density and high bandwidth communication, which plays a critical role for various data-centric applications. Despite growing number of research efforts on various aspects of monolithic 3D integration, commercial monolithic 3D ICs do not yet exist. This tutorial brief provides a concise overview of monolithic 3D technology, highlighting important results and future prospects. Several applications that can potentially benefit from this technology are also discussed.