  The field of planet formation is in an exciting era, where recent
observations of disks around low- to intermediate-mass stars made with state of
the art interferometers and high-contrast optical and IR facilities have
revealed a diversity of substructures, some possibly planet-related. It is
therefore important to understand the physical and chemical nature of the
protoplanetary building blocks, as well as their spatial distribution, to
better understand planet formation. Since PPVI, the field has seen tremendous
improvements in observational capabilities, enabling both surveys of large
samples of disks and high resolution imaging studies of a few bright disks.
Improvements in data quality and sample size have, however, opened up many
fundamental questions about properties such as the mass budget of disks, its
spatial distribution, and its radial extent. Moreover, the vertical structure
of disks has been studied in greater detail with spatially resolved
observations, providing new insights on vertical layering and temperature
stratification, yet also bringing rise to questions about other properties,
such as material transport and viscosity. Each one of these properties - disk
mass, surface density distribution, outer radius, vertical extent, temperature
structure, and transport - is of fundamental interest as they collectively set
the stage for disk evolution and corresponding planet formation theories. In
this chapter, we will review our understanding of the fundamental properties of
disks including the relevant observational techniques to probe their nature,
modelling methods, and the respective caveats. Finally, we discuss the
implications for theories of disk evolution and planet formation underlining
what new questions have since arisen as our observational facilities have
improved.
