This chapter surveys the experimental methods of measuring partition coefficients, log P and log D. It contains a description of the Dyrssen dual-phase potentiometric method, that truly is the ‘gold standard’ method for measuring log P of ionizable molecules, having the unique ten-orders of magnitude range (log P from −2 to +8). High-throughput methods are also described. A list of ‘gold standard’ quality partition coeffients is compiled.