The aim of this book is to present new results in rapidly developing fields of non-linear optics such as atomic coherence, magneto-optics of atomic media, Doppler-free spectroscopy, time-domain spectroscopy of dense atomic vapours, frequency comb spectroscopy and atom-surface interactions. The discovery of such coherent effects as coherent population trapping (CPT), electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) opened a new avenue for development in many fields of non-linear optics. Investigations during the last decade have shown many related new results and approaches in non-linear optics. These include non-linear magneto-optical processes (NMOP) in the context of high precision magnetometry, quantum information storage and processing, magnetic resonance imaging and magnetic particle detection; the use of ultra-thin (nanometric length) atomic vapour cells in magneto-optical experiments, which show new peculiarities of both coherent effects and NMOP; the use of frequency and amplitude modulated laser beams for NMOP studies in atomic vapours with the aim of extending the range of ultra-precise magnetometers to stronger, particularly geomagnetic fields; application of the counter-propagating beam technique to reveal the Doppler free non-linear Faraday rotation signals, as a spectroscopic tool and for applications; enhanced nonlinearly due to atomic coherence in multi-state systems; experimental study of EIT in solids; the use of the phase stabilised frequency domain combs of femtosecond laser pulses for velocity selective optical pumping of atomic hyperfine levels, as a direct frequency comb spectroscopy; time-domain non-linear spectroscopy of dense atomic vapours; atom-surface interactions and laser induced adsorption and description of alkali atoms on the surface of solid materials for nanotechnology applications.