Heap formation occurs widely in industry when particulate materials are filled into vessels such as hoppers, silos, bulk bags and bins. If the bulk materials are composed of particles with different size, shape, material density or surface properties, they tend to segregate spatially during heap formation. In most industrial settings, heap segregation is unwanted, because the inhomogeneity of the composition is detrimental for product quality and also can potentially impact subsequential processes. In this paper, we review previous efforts on understanding physical mechanisms and developing predictive models for heap segregation in the past a few decades. We also aim to provide a general picture of what have been learned from previous experimental, computational, and theoretical work and what are still missing for understanding and predicting heap segregation. (C) 2017 Elsevier B.V. All rights reserved.