This paper attempts to introduce the work described in this special section on colloid transport within a more general perspective of the evolution of our understanding of the importance of colloids in subsurface systems. The focus will be on the transport of colloidal particles in natural (i.e., chemically and physically heterogeneous) geological settings because the complexity imposed by these situations represents the greatest challenge to current and future understanding. Great progress has been made in addressing many of the key questions related to colloid transport. However, as in most areas of science, increased knowledge also serves to reveal new and more complex challenges that must be addressed.

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