The aim of this paper is to describe student-student online coaching, defined as "an online service where a student gets support on a specific subject matter from a more experienced student". Student-student online coaching emphasizes learning a subject matter by giving a student the opportunity to get coached by a coach, i.e. a more experienced student. Online coaching is encouraged by an organization, but the control of learning is primarily in the hands of the student. An example of online coaching is described, i.e. math coaching by instant messaging. A key challenge for coaches is to interpret the students' competence level, despite that they often do not know the students beforehand, in order to coach on a level that is within their zone of proximal development. (Contains 1 table.)