In this book, Professor Dahl carries forward his logico-empirical studies of the conditions under which political systems change or are transformed from one type to another�. The focus here is upon the conditions under which systems he designates as closed hegemonies, inclusive hegemonies, or competitive oligarchies are likely to develop into polyarchies, or intermediate variations thereof� To this reviewer, the present book enhances Professor Dahl's status as political diagnostician and analyst. It exemplifies beautifully the function of theory in disentangling the elements of a problem and understanding the relations between them, as distinct from constructing general, logically consistent theories that do not explain variations and deviant cases in the real world, or devising 'practical' solutions without knowing whether or why they work. Dahl uses concepts to reveal and enlighten us as to hidden relationships in the data.