Laser-atom interactions create atomic coherence and large nonlinear atomic polarization. We investigate resonant laser-atom interactions to generate large nonlinearities and control them using magneto-optical fields. Coherent control of high-order susceptibilities and magneto-optical rotation are demonstrated. Experiments are supported by theoretical studies that effectively describe the observed phenomena. It is shown that a new coherent field, with polarization orthogonal to a weak signal field, can be parametrically generated via an all-resonant four-wave-mixing process. This is demonstrated in a double-ladder system having two intermediate states between a ground and an excited state. It is shown that the parametricgeneration process can be coherently controlled by coupling lasers and magnetic fields. It is theoretically established that the underlying physics is a resonant three-photon process with a wide domain of control parameters. Electromagnetically induced transparency (EIT), where absorption of a weak probe is suppressed via quantum interference, is demonstrated in a usual three-level ladder system. It is observed that in contrast with EIT in a usual ladder system, addition of a second channel helps to suppress the absorption of two weak probe fields in the double-ladder system. The resulting enhancement of transmission in two different channels is due to gain caused by three-photon processes. Coherent control is strongly limited by coherence lifetime, which is the inverse of the dephasing rate. A lambda-system, having two ground states coupled to a common excited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing perturbs the dark state. A new method for measuring the ground-state dephasing rate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated. While two laser fields in a lambda-system are resonant with their respective transitions, the atomic polarizations are very sensitive to an applied magnetic field. This effect can be used for optical magnetometry. The degree of sensitivity of the magnetometer is determined by two competing parameters–atomic density and laser intensity. It is shown experimentally that the optimal sensitivity reaches saturation, which is contrary to the idea that sensitivity increases indefinitely with an increase in the above parameters.