Bioelectrical impedance analysis (BIA) is a non-invasive and inexpensive technique for assessing body composition. Predictive models can estimate fluid volumes and fat-free mass but they depend on assumptions. Although less easy to interpret, raw data like phase angle are assumption-free. Phase angle relates to cellular health, with high values in healthy populations. BIA finds multiple clinical applications in nutrition, chronic diseases, and rehabilitation. Its implementation in clinical practice and research requires standardized protocols, and better reference material should be established, as argued in this review.