Differentiation practices are defined by the Scottish Office Education Department as the identification of, and effective provision for, a range of abilities in one classroom, such that pupils in a particular class need not study the same things at the same pace and in the same way at all times. This issue of the "Interchange" describes two studies of differentiation practices in primary and secondary schools. The first study investigated how primary teachers allocate different work to different pupils and the extent to which they match the work to the pupils' achievement levels. The second study examined differentiation practices in four secondary school subject areas, mathematics, science, English, and modern languages, showing the pattern of differentiation in different subjects, the constraints on developing differentiation, and what pupils need to optimize their learning. Results indicated that differentiation practices can be successful when teachers share with pupils the management of their learning, promote the belief that achievement can improve, use a wide range of sources of information and support, identify a range of needs, and give and receive continuous feedback. (AP)