  Let $h$ be an instance of Gaussian Free Field in a planar domain. We study
level lines of $h$ starting from boundary points. We show that the level lines
are random continuous curves which are variants of SLE$_4$ path. We show that
the level lines with different heights satisfy the same monotonicity behavior
as the level lines of smooth functions. We prove that the time-reversal of the
level line coincides with the level line of $-h$. This implies that the
time-reversal of SLE$_4(\underline{\rho})$ process is still an
SLE$_4(\underline{\rho})$ process. We prove that the level lines satisfy
"target-independent" property. We also discuss the relation between Gaussian
Free Field and CLE$_4$.
