This paper provides a review on the flow of free-flowing coarse particles inside silos. Basic principles and elementary mechanisms involved in gravity flow of granular materials are discussed. The main focus is put on flow patterns, stress distribution, velocity profiles, mass flowrate, Residence Time Distribution (RTD) and segregation inside the silo. A summary of empirical correlations as well as a synthesis of more physical and mechanistic models related to these aspects are presented. Computational and numerical methods used to describe the granular flow of particles will be discussed in a separate paper. (C) 2018 Elsevier Ltd. All rights reserved.