The state of the art in the field of alkali chloride electrolysis for the production of chlorine, alkali hydroxide and hydrogen is characterized by the state of development of the membrane cell process. By 1996 about 230 membrane electrolysis plants with a combined NaOH capacity of about 11 million metric tons will be on line worldwide. This amounts to roughly one fourth of the world’s caustic soda production by electrolysis. In this review the main membrane cell technologies, the ion exchange membranes of different suppliers and anodes and cathodes of the membrane cells are described. This first part of the review ends with a look at the problems of brine quality for the membrane cells and a description of the products chlorine, caustic soda solution and hydrogen.