Recent metal additive manufacturing (AM) discoveries suggest many commonalities related to physical metallurgy principles developed for casting, welding, powder metallurgy, and thermal-mechanical processes. However, in-situ and ex-situ characterization has confirmed that the steady-state conditions assumed during welding are not strictly valid for the AM, because of spatial and temporal transients brought about by abrupt changes in energy delivery modes. Irrespective of these changes, recent results show that interface response function theories developed for directional solidification can be extended to predict the microstructural heterogeneity in direct energy deposition, selective electron beam melting, and laser powder bed melting process. In this talk, we will discuss successful examples of predicting the transition from columnar to equiaxed transition and primary dendrite arm spacing will be discussed. However, some of the challenges involved in estimating fundamental physical parameters for multicomponent alloys will also be stressed.