  The Sun is the only star that we can spatially resolve and it can be regarded
as a fundamental plasma laboratory of astrophysics. The solar transition region
(TR), the layer between the solar chromosphere and corona, plays an important
role in solar wind origin and coronal heating. Recent high-resolution
observations made by SOHO, TRACE, and Hinode indicate that the TR is highly
nonuniform and magnetically structured. Through a combination of spectroscopic
observations and magnetic field extrapolations, the TR magnetic structures and
plasma properties have been found to be different in coronal holes and in the
quiet Sun. In active regions, the TR density and temperature structures also
differ in sunspots and the surrounding plage regions. Although the TR is
believed to be a dynamic layer, quasi-steady flows lasting from several hours
to several days are often present in the quiet Sun, coronal holes, and active
regions, indicating some kind of plasma circulation/convection in the TR and
corona. The emission of hydrogen Lyman lines, which originates from the lower
TR, has also been intensively investigated in the recent past. Observations
show clearly that the flows and dynamics in the middle and upper TR can greatly
modify the Lyman line profiles.
