In many health professions, experienced practitioners assess students’ or inexperienced practitioners’ clinical skills but do not formally or explicitly rate their interpersonal skills, even though it is often suggested that failing or struggling students have poor interpersonal skills. The Interpersonal Skills Profile (ISP) has been widely used in UK health care programmes. The tool allows assessors to select five statements from a list, which they feel reflect the student’s achievement. These are usually graded from fail to excellent.Using a Realistic Evaluation approach this study examined how the ISP was used to assess interpersonal skills in a university pre-registration nursing programme. The use of the ISP was investigated through interviews with clinical nursing mentors, practice education facilitators and education champions as well as a documentary analysis of student assessment booklets. The findings led to the development of three middle range theories which may be useful in other contexts. These focus on: 1) overt assessment of interpersonal skills, 2) providing support to mentors and 3) feedback and ‘feeding-forward’ to students.