We review recent research on nonlinear optical interactions in waveguides with sub-micron transverse dimensions, which are termed photonic nanowires. Such nanowaveguides, fabricated from glasses or semiconductors, provide the maximal confinement of light for index guiding structures enabling large enhancement of nonlinear interactions and group-velocity dispersion engineering. The combination of these two properties make photonic nanowires ideally suited for many nonlinear optical applications including the generation of single-cycle pulses and optical processing with sub-mW powers.