The chapter “Fundaments of photoelectric readout of spin states in diamond” deals with the detection of NV centre spins in diamond using the photoelectric detection of magnetic resonances (PDMR) method, introduced in a series of recent publications. It provides in particular insights into the physics of electronic transitions of the NV center, leading to the free carrier generation, and discusses methodologies how to implement the photocurrent detection principles in the dynamically evolving field of quantum technologies. Recent results on the single electron and the single nuclear spin qubits photoelectric readout are presented, along with a microwave-free NV magnetometry technique. The chapter addresses suitable diamond materials for achieving high spin contrast and photoelectric detection rate, leading to effective spin detection.