The giant cells of certain algae are of especial value as experimental material for the investigation of physiological problems. This 1975 account gives a historical background to this. The authors consider water relations, ionic relations, the electrical properties of membranes, action potentials, active transport, carbon dioxide and bicarbonate ion transport and use, and protoplasmic streaming. The authors are at pains to point out the general implications of the findings for the plant kingdom and occasionally for the animal kingdom as well. Advanced students, teachers and research workers in plant physiology, cell physiology and biophysics will find this a stimulating account of an important area of research.