  Once the first sources have formed, their mass deposition, energy injection
and emitted radiation can deeply affect the subsequent galaxy formation process
and influence the evolution of the IGM via a number of so-called feedback
effects. The word 'feedback' is by far one of the most used in modern
cosmology, where it is applied to a vast range of situations and astrophysical
objects. Generally speaking, the concept of feedback invokes a back reaction of
a process on itself or on the causes that have produced it. The character of
feedback can be either negative or positive. Here, I will review the present
status of investigation of the feedback effects from the first stars and
galaxies.
