Circumstellar disks form during the early stages of star formation.
They play a key role in the formation process of the central star and
in the development of planetary systems. In this talk I will review
our current understanding of the structure, properties and evolution
of circumstellar disks. I will discuss how the study of disks can be
used to obtain insights on the stellar formation process and how we
can probe the earliest stages of planets formation in protoplanetary
disks.