The document provides an overview of the Biopharmaceutics Classification System (BCS). The BCS is a scientific framework that classifies drug substances based on their aqueous solubility and intestinal permeability. It includes four key classes based on whether a drug is highly soluble/permeable, low soluble/high permeable, etc. The BCS aims to predict a drug's absorption and bioavailability based on these characteristics, and provides guidance to drug regulators on biowaiver requests for in vivo bioequivalence studies.