We present a comprehensive study of the transmission characteristics of all-fiber Sagnac interferometers (FSIs) based on fiber Bragg gratings (FBGs). Analytic and numerical models have been developed for the design and characterization of FBG FSIs that incorporate gratings of arbitrary fringe structure. The transmission, phase, and time delay responses of several representative configurations that incorporate uniform-period and chirped gratings with and without apodization have been investigated theoretically and experimentally. Excellent agreement between the theoretical results and real device characteristics has been found in all cases. Our study clearly reveals that fiber grating-based FSIs offer potentially significant practical advantages not only for conversion of the reflective response of the grating into a transmissive response without loss but also by providing near-zero dispersion in the transmission bands, which offers attractive prospects for filtering components in high-speed wavelength-division multiplexing transmission systems.