Powder-based Directed Energy Deposition (DED) is one of the additive manufacturing processes, which allows users to manufacture complex parts through melting blown powders with a laser beam. DED has been used for repairing and cladding operations due to its near-net shape and high deposition rate advantage. In this presentation, a review of various theoretical models and numerical simulations, mostly via computational fluid dynamics (CFD), is conducted for melt-pool during DED deposition, including all the phenomena such as laser powder interaction, powder melt pool interaction, and melt pool fluid flow. In particular, powder interacts with a laser through absorption and scattering of electromagnetic radiation, leading to laser attenuation. When powder touches the melt pool, surface tension characteristic affects the lifting of a particle from the melt pool due to temperature difference. Melt-pool phenomena such as liquid metal flow driven by surface tension occur due to a temperature gradient on the surface of the melt pool. Therefore, it is critical to understand the fundamental physical phenomena during deposition and its relationship with process parameters, in order to produce optimized parts with desired properties through powder-based DED, such that modeling and numerical simulation play a key role in providing an efficient way of obtaining insights into these physical phenomena in the powder-based DED process.