Abstract: A high dimensional dynamical system is often studied by experimentalists through the measurement of a relatively low number of different quantities, called an observation. Following this idea, for a measure preserving system, we study Poincaré recurrence for the observation. We link the return time for the observation and the Hausdorff dimension of the image of the invariant measure. Then, we consider an application of these theorems: the recurrence for random dynamical systems. Finally, we show some results of recurrence for flows and in particular for the geodesic flow.