The studies of planet formation have been heavily dominated by theoretical work because observing planets in formation was not possible. The situation is however gradually changing. Thanks to increasingly powerful observing facilities and techniques, we are now able to peer into the birthplaces of planets – so-called circumstellar disks – and routinely find signatures hinting at, and for a few systems direct evidence of, on-going planet formation. In this talk I will introduce how planet formation theories are being tested with and improved by observational data. In particular, I will present a specific example PDS 70, a 5-million-year-old young star surrounded by a disk of gas and dust with two accreting giant protoplanets detected across a broad range of wavelengths from optical to millimeter, offering a perfect test bed for planet-disk interaction and planet formation theories.