Describing multi-component adsorption is fundamental to using sorption in any chemical separation. 50 years ago, Myers and Prausnitz made a seminal contribution to characterization and prediction of multi-component adsorption by introducing Ideal Adsorbed Solution Theory (IAST). Here, we give an overview of IAST, highlighting its continued role as a benchmark method in describing adsorption using illustrative examples from a variety of experimental and molecular modeling studies. (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 2757-2762, 2015