Circuitscape is an open-source Julia program that uses circuit theory to model connectivity in heterogeneous landscapes. Its most common applications include modeling movement and gene flow of plants and animals, as well as identifying areas important for connectivity conservation. Circuit theory complements commonly-used connectivity models because of its connections to random walk theory and its ability to simultaneously evaluate contributions of multiple dispersal pathways. Landscapes are represented as conductive surfaces, with low resistances assigned to landscape features types that are most permeable to movement or best promote gene flow, and high resistances assigned to movement barriers. Effective resistances, current flow, and voltages calculated across the landscapes can then be related to ecological processes, such as individual movement and gene flow. More detail about the underlying model, its parameterization, and potential applications in ecology, evolution, and conservation planning can be found in McRae (2006) and McRae et al. (2008).