The chapter discusses the interaction between theory and practice in solidification processing, highlighting recent advancements and the challenges faced. Key areas of progress include dendrite arm spacing, grain size control, columnar structures, eutectic-like in-situ composites, inclusion formation, macrosegregation, and non-dendritic structures. The author emphasizes three main ideas: the effective interaction between theory and practice, the need to consider different size scales simultaneously, and the complexity of many practical and fundamental problems. The chapter also delves into the structures encountered during solidification, such as equiaxed dendrites, columnar dendrites, and cellular structures, and discusses the conditions under which these structures form. Additionally, it covers the formation of inclusions and the impact of cooling rates on the resulting solid structures.The chapter discusses the interaction between theory and practice in solidification processing, highlighting recent advancements and the challenges faced. Key areas of progress include dendrite arm spacing, grain size control, columnar structures, eutectic-like in-situ composites, inclusion formation, macrosegregation, and non-dendritic structures. The author emphasizes three main ideas: the effective interaction between theory and practice, the need to consider different size scales simultaneously, and the complexity of many practical and fundamental problems. The chapter also delves into the structures encountered during solidification, such as equiaxed dendrites, columnar dendrites, and cellular structures, and discusses the conditions under which these structures form. Additionally, it covers the formation of inclusions and the impact of cooling rates on the resulting solid structures.