<p>In this paper, the plasmonic directional couplers (PDCs) are studied as key elements of nanophotonics circuits because of their potential applications for achieving strong light-matter interactions and subwavelength optical confinement, along with small size and compactness for on-chip applications. In the last two decades, three major types of PDCs have been identified according to the classification of these designs: plasmonic couplers in conventional platforms, hybrid systems, and periodic couplers. This paper discusses the functioning of PDCs based on numerical and experimental studies and describes them using the methods of coupled-mode theory and supermodes analysis. Typical Metal-Insulator- Metal (MIM) couplers allow for an extremely small size but suffer from inherent ohmic loss effects. Hybrid dielectric-plasmonic structures provide improved waveguiding properties along with larger bandwidths. The implementation of tapered-funnel, triangular, semi-elliptic, and oblique slits designs has resulted in significantly reduced impedance misalignment and provided coupler efficiencies exceeding 90%. Furthermore, Advanced materials such as graphene and ferroelectric films (barium titanate, lead zirconate titanate, and lithium niobate) offer tuning and electro-optic modulating ability needed for future photonic integrated circuits. Moreover, new uses of quantum dot-plasmonic complexes (PDCs) in quantum Plasmonics, optical switches, biosensors, and all-optical logic devices are described in the document. Finally, approaches to the development of high-speed optical and terahertz interconnects are covered, involving topological Plasmonic and integration on CMOS substrates.</p>