  The aim of this paper is to study the Dirichlet-to-Neumann operators in the
context of Dirichlet forms and especially to figure out their probabilistic
counterparts. Regarding irreducible Dirichlet forms, we will show that the
Dirichlet-to-Neumann operators for them are associated with the trace Dirichlet
forms corresponding to the time changed processes on the boundary. Furthermore,
the Dirichlet-to-Neumann operators for perturbations of Dirichlet forms will be
also explored. It turns out that for typical cases such a Dirichlet-to-Neumann
operator corresponds to a quasi-regular positivity preserving (symmetric)
coercive form, so that there exists a family of Markov processes associated
with it via Doob's $h$-transformations.
