Clients display considerable variations in functioning across the contexts that encompass their social environments (e.g., home, school/workplace, peer interactions). No single measurement method can fully capture these variations. Yet, assessors must balance the need to accurately capture clients' clinical presentations, and at the same time attend to clinical feasibility when considering which measures to use. We describe one method--informants' subjective reports about clients' mental health--and highlight research and theory that connects scores on informants' reports with scores on non-subjective methods (e.g., observed behavior, performance-based tasks, official records). We also discuss issues in use and interpretation of informants' reports, provide recommendations for clinical assessments, and outline important directions for future research and practice. [This chapter will be published in: G. Asmundson (Ed.), "Comprehensive Clinical Psychology, 2nd ed." New York, NY: Elsevier.]