Circumstellar disks from early accretion stages to debris disks motivate an active theoretical and observational research. Their physics is quite complex and implies the study of various domains such as the physics of grains, radiative transfert at various wavelengths, collisional and radiative destructive processes, dynamics, long term evolution... Such disks provide valuable information on the conditions of exoplanet formation, but also on the interaction between disks and already formed planets; they may be in some cases the indicators of the presence of planets or, on contrary, they may prevent from direct planet detections depending on the observational technique. We review the current most critical issues relative to disk studies, and discuss the need and interest for various observational techniques. We conclude on the importance for high dynamic observations and typical specifications.