Accretion disks are pivotal elements in the formation and earlyevolution of solar-like stars. On top of supplying the raw material for stellarbuild-up, their internal conditions also regulate the formation of planets. Theirstudy therefore holds the key to solve the mystery of the formation of our So-lar System. This article focuses on observational studies of circumstellar disksassociated with pre-main sequence solar-like stars, and presents a few selectedproblems where ALMA will contribute in finding answers. At optical and near-infrared wavelengths, the direct measurement of disk pa-rameters poses an obvious challenge: at the distance of typical star formingregions (e.g. -440 pc for Taurus), a planetary system like ours (with a diam-eter of 50 AU out to Pluto, but excluding the Kuiper belt) subtends only0'.'35. Moreover, its surface brightness is low in comparison to the bright centralstar. Hence, high angular resolution and high contrast imaging techniques arerequired if one hopes to resolve and measure such protoplanetary disks. Fortu-nately, potent imaging instruments have been available for about 10 years now.They cover a large part of the electromagnetic spectrum, from the UV/opticalwith HST, the near-infrared with ground-based adaptive optics systems to themillimeter range with long-baseline radio interferometers. It is therefore notsurprising that our knowledge of the structure of disks surrounding low-massstars has made a gigantic leap forward in the last decade. However, the angularresolution of current millimeter interferometers will remain significantly poorer than the resolution that is available at shorter wavelengths(~0"1) until ALMAprovides the necessary long baselines. At that time, astronomers will have ac-cess to data of comparable resolution. over a very large wavelength range, withunprecedented sensitivity. As a direct consequence, our understanding of thedisk structure and evolution should improve just as much. In the following pages I will attempt to give an overview of the structural andphysical parameters of protoplanetary disks that can be estimated today fromdirect observations.
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