Clark-type platinum electrodes remain one of the most accurate and reliable tools for measuring the partial pressure of oxygen (pO₂) in biological solutions. This review summarizes the physicochemical principles underlying the operation of these electrodes, including the electrochemical reduction of oxygen, diffusion-limited processes, and the role of the membrane in stabilizing the signal. The structural design of Clark-type sensors, calibration procedures, temperature and hydrodynamic requirements, and major sources of measurement error are described. Particular attention is given to practical aspects of real-time oxygen monitoring during biological experiments and to the applications of this method in studies of cellular respiration, mitochondrial function, hypoxia, and pharmacological effects. The review is complemented by a schematic representation of the electrode and an example of a typical polarographic recording, illustrating both the functional principle and the time-dependent dynamics of pO₂. Despite the emergence of modern optical oxygen sensors, Clark-type electrodes maintain their central role due to their simplicity, high sensitivity, and ability to provide direct and continuous control of oxygen levels in biological systems.