Due to their many advantageous properties,nanomaterials (NMs) have been utilized in diverse consumer goods,industrial products,and for therapeutic purposes.This situation leads to a constant risk of exposure and uptake by the human body,which are highly dependent on nanomaterial size.Consequently,an improved understanding of the interactions between different sizes ofnanomaterials and biological systems is needed to design safer and more clinically relevant nano systems.We discuss the sizedependent effects of nanomaterials in living organisms.Upon entry into biological systems,nanomaterials can translocate biological barriers,distribute to various tissues and elicit different toxic effects on organs,based on their size and location.The association of nanomaterial size with physiological structures within organs determines the site of accumulation of nanoparticles.In general,nanomaterials smaller than 20 nm tend to accumulate in the kidney while nanomaterials between 20 and 100 nm preferentially deposit in the liver.After accumulating in organs,nanomaterials can induce inflammation,damage structural integrity and ultimately result in organ dysfunction,which helps better understand the size-dependent dynamic processes and toxicity of nanomaterials in organisms.The enhanced permeability and retention effect of nanomaterials and the utility of this phenomenon in tumor therapy are also highlighted.