THE ACADEMIC YEAR 1965-66 will be of considerable significance in the history of education in Czechoslovakia. It saw the first large-scale, fully-sanctioned experiment in differentiation within the bounds of the basic 9-year school, the school which, incidentally, spans the whole period of compulsory general education. It will be necessary, first of all, to define 'differentiation'. Basically, of course, it can mean the division of pupils or students into groups for any educational purpose or purpose arising directly from the educational process. It is therefore obvious that certain kinds of differentiation, in this broad sense, are accepted in almost every educational system. Firstly, for instance, children are divided into groups or classes of convenient (or, very often, inconvenient) numbers for subject teaching. They are usually differentiated according to age and almost always, for certain subjects-P.E. and Domestic Science, for instanceaccording to sex. These kinds of differentiation are widely accepted. Secondly, however, children are in some countries divided for purposes of education according to race or colour. In England, richer parents may buy their children what they suppose to be a superior kind of education in the public schools. Both of these kinds of differentiation might be better known as segregation. Thirdly, children are selected for special educational treatment according to ability, attainment or interest. Clearly, it is not the mere fact of differentiation which gives rise to dispute but the principles upon which children are differentiated. Further, children may be divided according to several different principles at the same time.