Observers of education often assume measuring non-continuation -- or, more colloquially, "drop-out" rates -- among students must be straightforward. In fact, there are numerous different ways to measure the phenomenon of non-continuation that make it challenging to discuss. This paper addresses the following questions regarding non-continuation of students in the United Kingdom: (1) How good is the evidence base? (2) What is the scale of the problem? (3) What is the current picture? (4) What else is known about this topic? (5) What other factors need to be accounted for? and (6) What can be done?