This study examines how teachers of incarcerated youth conceptualize the purposes for assessment for the students they serve, as well as challenges in assessing students and using assessment data. As a case study of a high school within a maximum-security juvenile detention facility, it includes interviews with teachers, administrators, and a social worker; meetings with a team of teachers; and review of materials related to student assessment. The study found that teachers encounter student-level challenges, structural challenges, and assessment-specific challenges. Teachers view assessment as most effectively carried out through conversations with individual students. They are skeptical of the standardized individual assessments the school has adopted and the assessment data with which they are provided. The study concludes that teachers' emphasis on relationships with students can contribute to effective assessment of students' knowledge and skills; however, this will require selection of appropriate assessment instruments and professional development in using them effectively.