Some criteria of selfadjointness for unbounded operators in Hilbert spaces

This is a detailed presentation of some criteria of selfadjointness for unbounded operators in a Hilbert space, through operator Cauchy problems. We also include detailed preliminary results on unbounded linear operators in Hilbert spaces, the spectral theory of selfadjoint operators in Hilbert spaces, as well as the theory of extensions of Hermitian operators. The material of this thesis is classical, it was presented in the Operator Theory Seminar during the last two years, and contains material that can be found scattered through the textbooks cited in the bibliography list.