Prediction of events is the challenge in many different disciplines, from meteorology to finance; the more this task is difficult, the more a system is complex. Nevertheless, even according to this restricted definition, a general consensus on what should be the correct indicator for complexity is still not reached. In particular, this characterization is still lacking for systems whose time evolution is influenced by factors which are not under control and appear as random parameters or random noise. We show in this paper how to find the correct indicators for complexity in the information theory context. The crucial point is that the answer is twofold depending on the fact that the random parameters are measurable or not. The content of this apparently trivial observation has been often ignored in literature leading to paradoxical results. Predictability is obviously larger when the random parameters are measurable, nevertheless, in the contrary case, predictability improves when the unknown random parameters are time correlated.In a number of systems the dynamics is influenced by uncontrolled parameters which are intrinsically random or cannot be predicted with necessary precision. The evolution of a system of this type is described in the framework of random dynamical systems, word which indicates in the present paper also dynamical systems with noise.A dynamical system can be eventually studied by means of the associated symbolic dynamics, which, in this case, correspond to a stochastic process with random conditional probabilities, i.e. probabilities which depend on the same stochastic parameters.The obvious thing is that the possibility of forecasting the future evolution strongly depends on the possibility of measuring the parameters. The same model will have a different complexity (predictability) according to the fact that the measure is feasible or not. Even if the content this observation appears trivial it is often ignored in literature. For example, most frequently it is used a definition of complexity which considers the separation of nearby trajectories [1][2][3] under the same realization of the noise. This definition implicitly assumes that the realization is known and should not be used when the contrary happens, has often it is. For example, the phenomenon of noise induced order [3] should be not considered a reduction of complexity when the random disturbance is non measurable.A better characterization of complexity for dynamical systems with unmeasurable randomness has been recently found out for many physically relevant cases [4][5][6][7].In this paper we show how to find proper indicators of complexity in the two cases of measurable (accessible information) and non measurable (inaccessible information) randomness. We also show, with an example, that in case the stochastic parameters have memory, part of the inaccessible information is encoded in the dynamics of the system and can be recovered. In other words,