The book describes the current state-of-the-art of probe diagnostics, encompassing the latest achievements in electric probe science and technology. Beginning with an overview of plasma probe diagnostics, it subsequently goes beyond the single Langmuir probes which have received tremendous attention over the last few decades to cover recent developments. In particular, new probe theories and corresponding device implementations have been devised that allow reliable measurements in simple plasmas and more complex scenarios such as high-pressure plasmas, plasmas in strong magnetic fields, as well as anisotropic plasmas. It also covers new probe designs that have made it possible to reliably measure parameter fluctuations, particle fluxes, and energy in magnetized plasmas. Particular attention is paid to error analysis in probe measurements which can significantly increase their reliability and accuracy.