Optical spectroscopy is a well-developed scientific method to retrieve atmospheric constituents. A simple methodology is to study the interactions of a light source with the atmosphere. Examples of the interactions include absorption and scattering effect due to air molecules and aerosols. Depending on the trace gases or aerosols intended to be measured, different methodologies are applied. Differential Optical Absorption Spectroscopy (DOAS) is widely employed for the trace gas retrieval; whereas analysis of scattering process is used to determine the aerosol features. Experimentally, to retrieve nitrogen dioxide, either direct sunlight or an artificial broadband light source can be used, and the DOAS method is used to analyze the residual light intensity. To retrieve water vapor column densities, the Langley method is employed. In addition, to characterize aerosol features, diffuse sunlight is analyzed. Sunlight is an elegant broadband light source for the atmospheric interactions. With the application of the DOAS method and solar spectroscopy, the book presents the working principles, inversion algorithms, and instrumentation in atmospheric research and trace gas monitoring.