Additive manufacturing (AM) is a new paradigm for the design and production of high-performance components for aerospace, medical, energy, and automotive applications. The first chapter is a review article, that provides a comprehensive classification of directed energy deposition (DED) systems, process variables, process physics, modelling efforts, common defects, mechanical properties of DED parts, and quality control methods. To provide a practical framework to print different materials using DED, a process map using the linear heat input and powder feed rate as variables is constructed. Based on the process map, three different areas that are not optimized for DED are identified. These areas correspond to the formation of a lack of fusion, keyholing, and mixed mode porosity in the printed parts. In the final part of the paper, emerging applications of DED from repairing damaged parts to bulk combinatorial alloys design are discussed. In the second chapter, the process physics and solidification parameters during AM is studied in more detail. Understanding the dynamic solidification behaviour during metal additive manufacturing (AM) is essential, as it directly influences final microstructures and hence mechanical properties of the part. Many efforts in this domain are based on numerical and computational approaches. Some studies focus on experimental determination of these parameters using in-situ monitoring including pyrometry, IR imaging or synchrotron x-ray imaging. However, these methods cannot directly measure the changes in lattice parameters in real-time, which is necessary for estimating transient parameters like thermal gradient and cooling rates. Here, we demonstrate a novel approach using in-situ synchrotron x-ray diffraction to accurately measure solidification parameters during a single line scan AM of Inconel 625. Using this approach, we calculate the melt pool and mushy zone dimensions, along with real-time thermal gradient, cooling rate and solidification front velocity. We also estimate the dendritic arm spacings and the grain structure expected in the part. The effect of processing conditions on solidification parameters are discussed and compared with other approaches in the literature.