In this thesis, we construct the fundamental solutions of pseudo-differential operators q(x,t,D) with time-dependent negative definite symbols q(x,t,xi) which are discussed as generators of Feller- and L2-sub-Markovian semigroups . The results are based on the Hille-Yosida theorem, the standard results of analytic semigroups of operators and the fundamental solutions of time-dependent parabolic equations
