The ability to visualize structural features of the brain and associated functional information has fueled a revolution in our understanding of the brain. The optical technique two-photon microscopy (2PM) is widely used to study individual neural circuits and blood vessel networks in vivo because it is minimally invasive and provides three-dimensional images with cellular resolution. There is rising interest from neuroscientists for the ability to extend the traditional imaging depth of 2PM, which is typically limited to ∼500 μm below the surface of the brain. In this dissertation, I detail the development of a novel laser source that enables deep-tissue in vivo multiphoton microscopy imaging of blood vessel networks and neurons. Using an excitation wavelength near 1,300 nm at which scattering in tissue is minimized, I demonstrate the ability to chronically study vascular morphology and dynamics as well as neuron morphology at imaging depths of 1 mm and beyond.