Ambient energy harvesting for power supply to low power hardware, like sensors in inaccessible environments, has garnered sustained interest over the last two decades. Common sources of ambient energy include vehicle vibrations and natural fluid flow. The latter is also proposed as a possible quiet alternative to conventional renewable energy systems like horizontal and vertical axis wind turbines. Various forms of flexible structures have been proposed and tested over years and found to be comparable to conventional systems on an energy density basis. These harvesters often rely on instability of the fluid-structure coupled system at increasing flow velocities. Thus, the design of these harvesters requires an understanding of the complex fluid-structure interaction inherent in them, which may include nonlinear effects. The inclusion of an electric circuit to scavenge the vibratory energy further transforms it into a three-way coupling problem. Despite laboratory scale verification of the potential of these systems, practical deployment has been deterred by the fluctuating nature of the natural fluid flows. Recent researches have sought to develop adaptive harvesters for such scenarios. Some progress has also been made in exploiting the spatio-temporal flow fluctuations beneficially for enhancing harvested power. This chapter summarizes the state-of-the-art in this regard and classifies the various approaches to tackling flow fluctuations.