  The theory of stellar evolution plays a central role in astrophysics as
stellar models are used to infer properties for Galactic and Extragalactic
stellar populations as well as exoplanetary systems. However, despite decades
of experience, stellar models still face major issues linked to transport
processes of chemicals and angular momentum. This review will focus on some of
the processes responsible for the most sizable uncertainties in stellar models
such as for example convection, rotation and mass loss. The presentation will
discuss their implementation, their impact on theoretical predictions and how
various observational constraints can help us gain insight on the physics
inside stars and face the current challenges of the theory of stellar
evolution.
