Halide perovskites are an important, emerging family of electronic materials which have generated significant scientific interest due to their long carrier lifetimes, facile deposition procedures, and tolerance of large numbers of crystallographic defects. These properties have enabled the lead halide perovskites to be successfully used in a variety of optoelectronic devices including solar cells, light-emitting diodes, and photodetectors. Due to the toxicity of lead and the instability of the lead halide materials, it would be highly desirable to replace the lead halide perovskites with other materials which also demonstrate defect tolerance and long carrier lifetimes. One time-tested approach for identifying promising alternative materials is to synthesize structural analogs; materials with the same structure but different atomic compositions. This dissertation describes my efforts making one family of structural analogs, the halide double perovskites, and evaluating their optoelectronic properties. These materials also adopt the perovskite structure but divide the 2+ charge on the lead site unevenly over multiple sites allowing for a much wider variety of elements to be incorporated into the perovskite lattice. I report the synthesis of the first bismuth bromide double perovskite, Cs2AgBiBr6, and demonstrate that, due to its similar electronic configuration and structure, it possesses many of the same properties that the lead halide perovskites have including a similarly sized bandgap and long carrier lifetime as well as having higher stability. However, unlike the lead perovskites this bismuth double perovskite possesses an indirect bandgap. I next develop a model that describes the atomistic origins of double perovskite band structures. Using a qualitative Linear Combination of Atomic Orbitals method, I describe how band dispersion and band extrema in double perovskites arise naturally from the translational symmetry of the double perovskite structure and detailed bonding interactions. This treatment accurately predicts the band edge positions of almost every known double perovskite and allows for immediate prediction of the electronic structure of new double perovskites from the orbitals of the constituent elements. While the initial work on double perovskites proved promising, the bandgaps of most double perovskites known to date are too large to be useful for optoelectronic devices, particularly solar cells. I demonstrate that the doping or complete substitution of Tl3+ into Cs2AgBiBr6 reduces the bandgap of the material drastically. The completely substituted material, Cs2AgTlBr6, has the smallest bandgap of any known halide perovskite and is much smaller than is typical for bromide perovskites. I show that this small bandgap arises from the metal-to-metal charge transfer nature of double perovskite band edge transitions which allows significant control over the bandgap size through proper choice of metal pairs. Additionally, I demonstrate that Cs2AgTlBr6 undergoes a slow defect reaction, spontaneously evolving Br2 to the atmosphere and n-type doping itself. This defect reaction is analogous to the oxygen-exchange reaction in oxides and is likely general to all halide perovskites. Finally, I show that the halogen exchange reaction we found in Cs2AgTlBr6 occurs in other double perovskites as well. By carefully monitoring the electronic conductivity of the double perovskite Cs2SnI6 I demonstrate that it also undergoes reversible loss and incorporation of I2 with the atmosphere. The kinetics of this reaction follow a one-dimensional diffusion model which can be used to extract diffusion coefficients for halide vacancies and the standard enthalpy of the halogen exchange reaction. Combining these results with ionic conductivity and Hall measurements I am able to construct a detailed defect chemical model. This model predicts the concentrations of major defect species in Cs2SnI6 from experimentally measured thermodynamic parameters. I also demonstrate that deliberate Ba2+ doping of Cs2SnI6 is able to greatly reduce the extent of the halogen exchange reaction and fix the electronic doping to a specific level.