Let $A$ be a normal operator in a Hilbert space $mathcal{H}$, and let $mathcal{G} subset mathcal{H}$ be a countable set of vectors. We investigate the relations between $A$, $mathcal{G}$ , and $L$ that makes the system of iterations ${A^ng: gin mathcal{G},;0leq n< L(g)}$ complete, Bessel, a basis, or a frame for $mathcal{H}$. The problem is motivated by the dynamical sampling problem and is connected to several topics in functional analysis, including, frame theory and spectral theory. It also has relations to topics in applied harmonic analysis including, wavelet theory and time-frequency analysis.