A preface should justify the existence of the book it precedes and this is invariably done in scientific texts by reference to the explosive growth of the field since the last such volume appeared. In molecular biology, most fields can be justifiably described as growing explosively, as should be the case for a young and vigorous science, but the study of membrane proteins stands out as one which has taken giant strides in the last few years. Ignorance of the structure and function of membrane proteins at the molecular level was certainly not due to lack of interest but rather was a result of lack of appropriate techniques. It has above all been the development of new experimental methods which has wrenched membrane biochemistry out of what Anthony Martonosi fetchingly called its 'romantic phase' (Le. lots of ideas and few facts), into an era when the determination of membrane protein structure and mechanism is a reasonable goal. Membrane proteins are generally classified as peripheral or integral. Peripheral proteins are relatively easily dissociated from membranes by mild treatments whence their study is essentially no different to that of soluble proteins. This book therefore concentrates on integral proteins which are strongly bound to the membrane by hydrophobic interactions with lipids. A crucial step in their study is of necessity the d~velopment of methods of solubilization and purification under non-denaturing conditions.