This book book contains lectures and tutorials offered to PhD students willing to learn the impact of stellar interferometry to its main scientific application : stellar physics. Indeed, stars are seen different, now that their surface or environments can be resolved by these incredibly complex systems that are long-baseline interferometers. Surface or environment images of a few targets start to be made, and will grow in the near-future with the advent of next generation imaging instruments. The becoming of this book was impulsed by the VLTI school in the town of Barcelonnette in the french Alpes. The organization committee decided to write down the courses delivered during this very complete school of astrophysics. This book is dedicated to the memory of one of these organizers: Olivier Chesneau, who drived many stellar physics projects at the Observatoire de la Côte d’Azur, and who disappeared before being able to see the result of this work.