  A majority of quasar surveys have been based on criteria which assume strong
blue continua or a UV-excess. Any amount of dust along the line-of-sight is
expected to drastically extinguish the optical/UV flux leading to a selection
bias. Radio surveys however should suffer no bias against extinction by dust.
Recently, a large complete sample of radio-selected quasars has become
available (the `Parkes sample'). A majority of these sources exhibit
optical--to--near-infrared continua that are exceedingly `red', very unlike
those of quasars selected optically. The purpose of this thesis, broadly
speaking, is to explore the problem of incompleteness in optical quasar surveys
due to obscuration by dust, and to interpret the relatively `red' continua
observed in the Parkes quasar sample. The first part of this thesis explores
the observational consequences of an intervening (foreground) cosmological dust
component, such as that located in galaxies and clusters. The second part
examines the continuum properties of Parkes-quasars in the framework of a
number of absorption and emission mechanisms to assess the importance of
extinction by dust. It is found that spectroscopic and photometric data for the
``reddest'' quasars (with B-K>5) is best explained by the dust model, rather
than ``purely'' by an intrinsic emission property.
