High quality electron beams are a key tool in much of modern physics, forming the backbone of electron diffraction and microscopy, free electron lasers, and several proposed particle colliders. The quality of an electron beam in the transverse dimensions, such as its size and divergence, can be summarized in a single quantity, the emittance, which sets, among other things, the resolution of electron microscopes, the limiting power of free electron lasers, and the luminosity of colliders. The emittance of the beam is limited by the emittance at the source, thus rendering high quality electron sources critical to modern accelerators. This dissertation discusses the characterization of the quality of a specific class of electron sources, the photocathode, which utilizes laser illumination to produce the electrons, and their use in two applications: ultrafast electron diffraction and free electron lasers. In particular, this dissertation describes the successful commissioning of the first ultrafast electron diffraction apparatus to utilize low emittance photocathodes at threshold, resulting in a combination of sub-10 micron probe sizes, sub-200 fs bunch lengths, and state-of-the-art spatial resolution. Finally, this dissertation reports the benefits that low emittance photocathodes could provide to a proposed new free electron laser and shows a novel method for ensuring that low emittances at the cathode are preserved throughout the beamline.