  A functional calculus approach is applied to the derivation of evolution
equations for the moments of the magnetization dynamics of systems subject to
stochastic fields. It allows us to derive a general framework for obtaining the
master equation for the stochastic magnetization dynamics, that is applied to
both, Markovian and non-Markovian dynamics. The formalism is applied for
studying different kinds of interactions, that are of practical relevance and
hierarchies of evolution equations for the moments of the distribution of the
magnetization are obtained. In each case, assumptions are spelled out, in order
to close the hierarchies. These closure assumptions are tested by extensive
numerical studies, that probe the validity of Gaussian or non--Gaussian closure
Ans\"atze.
