  Stochastic differential equations for processes with values in Hilbert spaces
are now largely used in the quantum theory of open systems. In this work we
present a class of such equations and discuss their main properties; moreover,
we explain how they are derived from purely quantum mechanical models, where
the dynamics is represented by a unitary evolution in a Hilbert space, and how
they are related to the theory of continual measurements. An essential tool is
an isomorphism between the bosonic Fock space and the Wiener space, which allow
to connect certain quantum objects with probabilistic ones.
