Designing polymers for improving solid-liquid and solid-solid separations in mineral processing requires a sound knowledge of the fundamentals of colloid stability, adsorbed polymers, and floc fort-nation. This article outlines these fundamentals, and illustrates the use of such principles in two examples: the design of red-mud flocculants for use in the Bayer process, and the design of polymeric depressants for use in mineral-specific separation by means of flotation. (C) 2003 Elsevier B.V. All rights reserved.