Modulation instability (MI) of a continuous wave (CW) background solution of the nonlinear Schrödinger (NLS) equation is a well-known phenomenon that occurs in a variety of fields, such as nonlinear optics, hydrodynamics, plasma physics and Bose-Einstein condensation [1, 2]. In the nonlinear optics context, MI is the main mechanism for the generation of optical solitons, supercontinuum (SC) [3, 4], and rogue waves [5]. MI may be induced either by quantum noise or by a weak seed signal [6]: in the latter case, the initial stage of exponential signal amplification is followed first by the generation of higher-order sideband pairs by cascade four-photon mixing processes. Next, nonlinear gain saturation occurs, owing to pump depletion. After the maximum level of pump depletion is reached, which depends on the initial sideband detuning, the pump power and the fiber dispersion, energy flows back from the sidebands into the pump, until the initial condition is recovered, and so on. This phenomen provides a classical example of the so-called Fermi-Pasta-Ulam (FPU) recurrence [7, 8]. This process can be described in terms of exact solutions of the NLS equation [9-12], and has been experimentally observed in different physical settings, such as deep water waves [13, 14], in nonlinear optical fibers [15-20], in nematic liquid crystals [21], magnetic film strip-based active feedback rings [22], and bimodal electrical transmission lines [23]. Important qualitative physical insight into the FPU recurrence dynamics (e.g., the existence of a homoclinic structure and the associated dependence of the FPU recurrence period upon the input relative phase between pump and initial sidebands) may be obtained by means of a truncation to a finite number of Fourier modes, which may lead to simple, low-dimensional models [24-26]. In Section 1.1, we present an overview of the analysis of the nonlinear dynamics of MI by means of a simple three-mode truncation. Next we discuss how the coupling between two polarization modes in a birefringent optical fiber may extend the domain of MI to the normal dispersion regime. Also in the vector case, an important qualitative