This chapter investigates the properties of the modes supported by three-dimensional subwavelength plasmonic slot waveguides. It shows that the fundamental mode supported by a symmetric plasmonic slot waveguide, composed of a subwavelength slot in a thin metallic film embedded in an infinite homogeneous dielectric, is always a bound mode. The chapter shows that for an asymmetric plasmonic slot waveguide, in which the surrounding dielectric media above and below the metal film are different, there always exists a cutoff wavelength above which the mode becomes leaky. It also investigates the properties of compact couplers between high-index contrast dielectric slab waveguides and two-dimensional metal-insulator-metal sub-wavelength plasmonic waveguides. At microwave frequencies, where metals do not have a plasmonic response, two-conductor waveguides are used to guide subwavelength modes. The chapter considers an asymmetric plasmonic slot waveguide structure in which the surrounding dielectric media above and below the metal film are different.