The present (cumulative) thesis examines fundamentals of nanostructure-enhanced extreme-ultraviolet light generation in noble gases using two different nanostructure geometries for local field-enhancement. Specifically, resonant antennas and tapered hollow waveguide nanostructures are utilized to enhance low-energy femtosecond laser pulses, which in turn induce light emission from excited xenon, argon and neon atoms and ions. Spectral analysis of this radiation reveals that coherent high-order harmonic generation is not feasible under the examined conditions, contrary to former expectations and reports. Instead, the spectral characteristics unequivocally identify that incoherent fluorescence from multiphoton excited and strong-field ionized gas atoms is the predominant process in such schemes. Furthermore, novel nanostructure-enhanced effects are reported such as surface-enhanced fifth-order harmonic generation (from bow-tie nanoantennas) and the formation of a bistable nanoplasma (in a hollow waveguide). These effects offer intriguing links between nonlinear nano-optics, plasma dynamics and extreme-ultraviolet radiation. v vi Contents List of Figures ix 1 Introduction