Recent developments in holographic recording of volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass have demonstrated their utility as components in high power laser systems for spectral narrowing, transverse mode control, beam combining, and pulse stretching/compression. VBG structures are capable of diffracting incident light into a single diffraction order with high efficiency given the Bragg condition is met. The Bragg condition depends on both the wavelength and angle of the incident light making VBGs useful for filtering and manipulating both the wavelength and angular spectrum of a source. This dissertation expands upon previous research in PTR VBGs by investigating multiplexed VBGs and their applications in laser systems. Multiplexing involves the integration of several VBGs into the same volume of PTR glass. This process enables the fabrication of splitting and combining elements which have been used for high power beam combining with significantly reduced complexity as compared to other combining schemes. Several configurations of multiplexed beam combiners were demonstrated for both spectral and coherent combining systems with high power results yielding a combined power of 420 W with 96% efficiency. Multiplexing was also used to produce unique phase structures within VBGs. This effect was exploited to create extremely narrowband spectral filters called moire Bragg gratings. The technical challenges of producing moire gratings in bulk glass have revealed new insights into the use of PTR glass as a recording medium and produced devices capable of narrowband filtering of only 15 pm in the near infrared. Experiments were performed using such devices as intra-cavity laser elements for longitudinal mode selection. Investigations have also been made into increasing the level of multiplexing possible within PTR glass. These explorations included scaling the number of beam combining channels, fabrication of integrated multi-notch filters, and generated several other potentially interesting devices for future research. The summation of this work indicates a promising future for multiplexed VBGs in PTR glass.