The property profile of modern alloys is determined by their microstructure, based on the chemical composition; the morphology, distribution and quantity of the phases; and the process history of the material. Melting, casting, and solidification are the starting points in classical alloy production. At known composition, phase diagrams can help to predict the phases present as a function of temperature under equilibrium conditions. However, during melting, alloying, casting, and solidification, equilibrium conditions are rarely encountered. Similar can be said about heat treatments and wrought processing. It can therefore be helpful to take a closer look at these dynamic processes to optimize processing steps and properties. In situ investigations using synchrotron radiation focusing on the solidification behavior of magnesium alloys clearly show the influence of solidification parameters on the formation and transformation of stable and metastable phases. This information can contribute to validate and/or to improve phase diagrams and suited predictions.