Stars are the building blocks of galaxies, clusters, associations, and planetary systems. Evolution of stars is driven by physical processes in their interiors making theory of stellar interior structure and evolution (SSE) an important ingredient of many studies in contemporary astrophysics. Modern high-precision space photometry has made it strikingly clear that large discrepancies occur between theory and observations, due to poor understanding of transport phenomena inside stars. The main reason is that the theoretical descriptions of stellar interiors lack a proper observational calibration. In this talk, I will provide an overview of recent results and steps that have been taken towards resolving the above major problem in stellar astrophysics. In particular, I will focus on such physical phenomena as mixing, rotation, and angular momentum transport inside stars that are massive enough to posses convective core when arriving on the Zero Age Main Sequence (ZAMS). I will touch upon the role of stellar seismic activity and magnetism in setting up physical conditions inside stars, and on how binarity may alter stellar evolution on the one hand and help calibrating models of interior structure and evolution of stars on the other hand. Finally, I will show that optimal opportunities to improve upon our understanding of inner workings of stars occur right now thanks to several amazing all-sky surveys that are either ongoing or being planned in the near future.​