Pneumatic conveying systems are used widely to convey different types of materials in a variety of industrial settings. The knowledge on flow modes is particularly important in the design stage of pneumatic conveying lines in terms of energy requirements. There are some classifications of flow modes valid for both powders and bulk solid particles based on mean particle size, particle and gas density and loose poured bulk density considering also air/particle interactions. In this paper, the available literature on classifications of flow modes for pneumatic conveying are summarized. The material and flow parameters which are defined separately with different symbols are also presented on a common base. The results are tabulated in terms of proposed critical equations in order to provide ease and understanding in practice.