Resonant leaky modes can be induced on dielectric, semiconductor, and metallic periodic layers patterned in one or two dimensions. In this paper, we summarize their physical basis and present their applicability in photonic devices and systems. The fundamental amplitude and phase response of this device class is presented by computed examples for TE and TM polarizations for lightly and heavily spatially modulated gratings. A summary of potential applications is provided followed by discussion of representative examples. In particular, we present a resonant polarizer enabled by a single periodic silicon layer operating across 200-nm bandwidth at normal incidence. Guided-mode resonance (GMR) biosensor technology is presented in which the dual-polarization capability of the fundamental resonance effect is applied to determine two unknowns in a biodetection experiment. In principle, using polarization and modal diversity, simultaneously collected data sets can be used to determine several relevant parameters in each channel of the sensor system; these results exemplify this unique capability of GMR sensor technology. Applying the GMR phase, we show an example of a half-wave retarder design operating across a 50-nm bandwidth at λ~1550 nm. Experimental results using a metal/dielectric design show that surface-plasmon resonance and leaky-mode resonance can coexist in the same device; the experimental results fit well with theoretical simulations.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.