This thesis studies the long time dynamics of resonant systems in the weakly nonlinear regime. It is divided into two main parts. In the first one, we consider the resonant equation, which captures the energy transfer between normal modes of the system. Different tools to extract analytic information from the resonant equation are developed. After that, we apply them to a large number of resonant models. Some of them consist of a scalar field in different geometries as well as the Gross-Pitaevskii equation. In the second part of this thesis, asymptotically anti-de Sitter geometries subject to time-periodic boundary conditions are studied. The phenomenology allowed by these conditions is explored through the environment of time-periodic geometries. In particular, we construct their phase-space and delimit the regions of linear stability. We also present a protocol to dynamically construct time-periodic geometries
