Outcomes assessment has been a mandate across higher education for at least a decade, and instruction librarians are no strangers to the call to show what their students have actually learned. Most libraries keep general statistics on instruction, including the number of class sessions librarians teach and the number of students in attendance. These numbers, however, reflect only "outputs" of an instruction program. Administrators call upon librarians (and other faculty) to measure student learning and the impact of specific programs on student retention. Educational theory and practice provide models for assessment, yet librarians face significant challenges in this environment. Librarians often teach only single sessions of courses conceived, created, and controlled by traditional teaching faculty members, with little control over course pedagogy or learning outcomes, and limited opportunities to assess students.