Loneliness—a subjectively sensed deficit in one’s social relationships—is most commonly assessed using static, standardized questionnaires where all respondents are presented with the exact same questions. Yet, loneliness is a subjective phenomenon with vast between-person differences regarding its predictors, correlates, and outcomes. In this chapter, we argue that prevailing approaches to measuring loneliness fall short of capturing this complexity—both within and between individuals. We outline three central limitations of current measurement approaches: (1) item content may miss the core experiential elements of individuals’ loneliness, (2) there is ambiguity around how many and which aspects of loneliness are captured in current instruments and how stable these aspects are across time and contexts, and (3) a predominant focus on between-person comparisons, which neglects within-person dynamics. These limitations not only obscure the phenomenon but also risk misinforming research and practice. To move forward, we propose three key opportunities: (1) adopt a more idiographic approach that takes the subjective nature of loneliness seriously by tailoring measurement to the individual; (2) embrace an iterative understanding of measurement, where items and scales are subject to ongoing refinement and adaptation; and (3) leverage advances in Item Response Theory (IRT) and computerized adaptive testing to develop flexible, efficient, and context-sensitive assessments. Taken together, these recommendations aim to reimagine loneliness measurement as a dynamic, person-centered process that can deepen the understanding of this complex phenomenon.