Over the last two decades the atomic coherence and quantum interference were extensively used in high resolution laser spectroscopy. As a matter of fact a new branch of spectroscopy, termed as ?coherent optical spectra? has been emerged to incorporate phase dependent effects on the spectral lines, as well as various nonlinear absorption processes. Apart from spectroscopic point of view, the coherency effects for some fascinating counter intuitive quantum optical phenomena, regarding wave propagation and hence on the control of optical properties of the medium is an active research subject. This book presents an analytical treatment for investigating the coherent effects in the domain of semiclassical atom-field interaction. Under the density matrix formalism different types of systems are considered to study the coherent lineshape and hence various quantum optical phenomena. In particular the influence of the closely spaced transitions on the quantum coherence and hence the subsequent effects are analyzed. The analysis should help and encourage the students of physics as well as potential readers, who want to pursue Quantum Optics in post graduate or research level.