Are planetary systems forming around young stars today?
Observations at infrared and longer wavelengths have done much in the past two decades to reveal the stages that interstellar gas and dust pass through along the way to forming stars and planetary systems. We now have observational evidence relating to most of the stages of star formation: dense cores that are on the verge of gravitational collapse in molecular clouds, newly-formed protostars characterized by continuing infall of gas and dust, young stellar objects associated with strong, bipolar winds suggesting that they are still accreting mass from the disks that surround them. Finally, we have observations of pre-main-sequence stars with evidence of lingering circumstellar disks. Such disks are observed in association with most, if not all, young stars. Disk properties can be deduced from observations at infrared to millimeter wavelengths, and millimeter-wave interferometers are beginning to probe their structure on scales relevant to planetary system formation.