Where, when and how do (gas giant) planets form? - Up to now the formation process of planets was mostly a subject of theroretical studies and computer simulations. Empirical data were only used to constrain the initial conditions (e.g., the physical and chemical conditions in circumstellar disks) or the outcome of the planet formation process (e.g., by comparing bulk density and atmopsheric composition from simulations to observed exoplanets or by comparing simulated planet populations to the observed population of exoplanets). Recently, however, high-contrast and high-spatial resolution observations revealed a few objects that are best explained with young planets that are still embedded in the circumstellar disks of their host stars. For the first time, these objects may allow us to derive empirical constraints on the immediate formation process of gas giant planets. In this talk, I will summarize the current knowledge of these systems, highlight the challenges that remain in interpreting the existing data and how they can be overcome, and illustrate why empirical constraints on the planet formation process are crucial for exoplanet research in general.