This dissertation presents subwavelength optics with focus on the theory and applications of subwavelength apertures in a metal film. Two main issues regarding the optics with subwavelength apertures are investigated. As the first issue, the extraordinary optical transmission (EOT) through a single hole in a metallic waveguide is presented. A total transmission through a single subwavelength aperture is theoretically predicted for a perfect electric conductor regardless of the aperture size, without relying on aperture arrays and surface corrugations as presented in previous works. The waveguide EOT is then applied to boost the optical throughput of an apertured near-field scanning optical microscope (NSOM) probe. Using a new structure for the apertured NSOM probe which allows for waveguide EOT, the optical throughput and the damage threshold are boosted by 100× and 40× as compared to a conventional structure, and the experimental findings are backed-up by comprehensive finite-difference time-domain (FDTD) simulations. Single fluorescent molecules are scanned using the EOT apertured NSOM probe, and a spatial resolution of 62 nm is achieved. As the second issue, subwavelength apertures are found useful for optical trapping. A small dielectric particle can significantly change the optical transmission through an aperture by dielectric loading, and subsequently, a large optical force is induced which favors trapping. A self-induced back-action (SIBA) optical trap is designed using a circular nanohole in a gold film. Trapping of 50 nm polystyrene particle is experimentally achieved, which is not possible using a conventional single beam optical tweezers. The circular nanohole SIBA trap works beyond the perturbative regime, as proven by FDTD simulations and a Maxwell stress tensor analysis. We further improve the nanohole trapping using a double-nanohole, which is more sensitive for small dielectric changes due to the intense local field enhancement between its two sharp tips. A single 12 nm silica sphere is experimentally trapped using the double-nanohole, as the smallest trapped dielectric particle reported. We also achieve the trapping of a single protein – a bovine serum albumin (BSA) protein with a hydrodynamic radius of 3.4 nm in the folded form. The trapped BSA is also unfolded by the large optical force, as confirmed by experiments with changing optical power and changing pH. The high signal-to-noise ratio of 33 in monitoring single protein trapping and unfolding shows a tremendous potential for using the double-nanohole as a sensor for protein binding events at a single molecule level. / Graduate