Patients with acute/critical illness are particularly vulnerable to muscle loss and fluid shifts, which adversely impact clinical outcomes. Assessment of these parameters in hospital settings is often subjective and imprecise, which creates discrepancies in identification and difficulty in assessing longitudinal changes. Body composition (BC) technologies provide objective information about muscle and fluid status that can enhance clinical assessment, and BC variables could be biomarkers for prognosis and targets to monitor intervention. There is growing interest in computed tomography (CT), ultrasound (US), and bioimpedance techniques as bedside assessment tools in clinical populations, and specific muscle measures, whole-body BC estimates, and select BC variables show promise as biomarkers of muscle health, nutrition risk, and fluid status. This brief review highlights work within the past 5 years on the use of BC variables generated from CT, US, and bioimpedance in clinical populations with an emphasis on those with acute/critical illness and a brief discussion of implementation challenges in these populations. Consensus on measurement protocols will facilitate identification of BC targets that best reflect prognosis and outcomes and will ultimately allow clinicians to identify individuals who would benefit most from targeted nutrition and physical therapy interventions and reliably monitor their response to treatment.