"shrink" the wavelength of light. The finite permittivity also allows for penetration of the field into the metal. These factors can increase the cut off so that apertures much smaller than half the wavelength can have propagating modes. [6] In addition to rectangular apertures, [7] various shapes have been investigated for having higher transmission such as c-shape, [8] bow-tie, [9] and double nanoholes. [10,11] The shaping of apertures allows for further localizing the field to a "hot spot," thereby reducing the interaction volume. Those works have shown that apertures in real metals can dramatically reduce the interaction volume to the single digit nanometer scale, comparable to single molecules or nanoparticles. [12] Metal nanostructures have been studied extensively for enhancing the interaction with single emitters. [14] These studies seem to be predominantly focused on isolated nanostructures made of metal, rather than apertures. Apertures in metal films offer several features that benefit interaction with single emitters (Figure 1): Nanoapertures in metal films have long been recognized for their ability to confine light to subwavelength dimensions. This allows for high-resolution imaging of single emitters with enhanced emission rates. It also provides a background-free environment that has been exploited for biophysical applications like fluorescence correlation studies at high physiological concentrations and DNA sequencing. Recently, other advantages of single apertures have been recognized, including optical trapping, thermal management, and nanofluidic (nanopore) functionality. This paper reviews metal nanoapertures and their applications to single emitter studies for biophysical and quantum applications.