While informal caregivers provide essential support, this vital role often subjects them to substantial demands, which can culminate in adverse outcomes for their own well-being. Conceptual ambiguity and the limitations of existing frameworks, particularly the application of occupational burnout models, hinder a clear understanding of caregiver distress. This integrative conceptual review addresses these gaps by proposing "informal caregiver exhaustion" as a distinct construct tailored to the unique context of informal caregiving. We define informal caregiver exhaustion as a multidimensional (physical, emotional, cognitive, social dysfunction/withdrawal), chronic state of resource depletion resulting from prolonged exposure to cumulative caregiving demands exceeding replenishment capacity. Informal caregiver exhaustion is differentiated from related concepts like job burnout, subjective burden, general fatigue, and stress. We propose an integrative theoretical framework based on an adapted Input-Process-Output (IPO) systems model. This framework conceptualizes informal caregiver exhaustion as an output emerging from the dynamic interplay of baseline inputs (e.g., caregiver characteristics, recipient needs, external supports) and caregiving processes (e.g., tasks, interactions, appraisals, coping), explicitly differentiating these components to address methodological challenges like confounding and selection bias. The review outlines pathways leading to informal caregiver exhaustion and details its consequences for caregiver well-being across the proposed dimensions. This conceptualization provides a robust foundation for future empirical research, including construct validation, longitudinal studies testing the IPO model, and the development of targeted measurement tools and interventions to better support informal caregivers.