In this paper, we review the basic properties of resonant leaky mode elements implemented with periodic waveguide layers and consider their applicability in photonic devices and systems. Leaky waveguide modes can be exited when an incident light beam is coupled into the waveguide structure through an inscribed periodicity under phase-matching conditions. This results in generation of a guided-mode resonance field response in the spectrum. Device operation can be explained in terms of the photonic band structure and associated leaky-wave effects near the second stop band. Resonant devices such as bandpass/bandstop filters, polarizers, wideband reflectors, biosensors, tunable filters, and display pixels can be designed using this operational principle.