The vast repertoire of electrical activity displayed by neurons, cardiac myocytes, and various endocrine and sensory cells is the result of membrane-bound ion channels each producing a distinct conductance that facilitates current flux through the membrane. These conductances may be static, or their magnitudes may be voltage or ligand dependent. Dynamic clamp, a term for the various combinations of software and hardware that simulate these conductances, has proven to be a valuable tool for electrophysiologists studying excitable cells. One can think of dynamic clamp as a tool for immersing neurons in virtual reality-inspired experiments. Virtual synapses may be simulated, allowing a biological neuron to communicate with or receive inputs from real and simulated cells. Dynamic clamp can also be used to introduce virtual voltage-gated channels into biological neurons, thus altering the neurons integrative properties. This Toolbox article includes a brief description of how conductances are simulated as well as a discussion of the neurophysiological applications of dynamic clamp and its potential pitfalls. The origins of dynamic clamp, its implementation, and its applications to cardiac and endocrine electrophysiology have been reviewed elsewhere (Sharp et al., 1993; Prinz et al., 2003; Goaillard and Marder, 2006; Wilders, 2006; Destexhe and Bal, 2009).