I observe Young Stellar Objects (YSOs) with infrared and X-ray observations, in order to investigate the properties and the evolution of protoplanetary disks around the young stars, and to study the stellar populations and star formation histories of young clusters and OB associations. I also investigate the feedback effects of massive stars on the surrounding clouds (i.e. cloud dispersal and triggering of star formation) in various star forming regions, in particular the Carina Nebula and the Scorpius-Centuarus OB association. Another important topic concerns environmental effects (in particular external UV and X-ray irradiation from neighboring stars) on the protoplanetary disks around young stars and their possible effects on the planet formation process.