Obesity is a problem of serious concern among chronic kidney disease (CKD) patients; it is a risk factor for progression to end-stage renal disease and its incidence and prevalence in dialysis patients exceeds those of the general population. Obesity, typically assessed with the simple metric of body mass index (BMI), is considered a mainstay for nutritional assessment in guidelines on nutrition in CKD. While regular BMI assessment in connection with the dialysis session is a simple and easy-to-use monitoring tool, such ease of access can lead to excess-of-use, as the value of this metric to health care professionals is overestimated. This review examines BMI as a clinical monitoring tool in CKD practice and offers a critical appraisal as to what a high or a low BMI may signify in this patient population. Topics discussed include the utility of BMI as a reflection of body size, body composition and body fat distribution, diagnostic versus prognostic performance, and consideration of temporal trends over single assessments.