This paper presents a comprehensive theoretical and experimental investigation of Photothermal Deflection Spectroscopy (PDS), a sensitive spectroscopy technique based on the photothermal deflection of a laser beam. The authors develop a general theoretical framework for both collinear and transverse PDS, considering both continuous wave (cw) and pulsed cases for solids, liquids, gases, and thin films. They derive expressions for the temperature distribution and optical beam propagation, and provide numerical evaluations to validate the theoretical predictions. The experimental setup includes configurations for transverse and collinear PDS, using various light sources and position sensors to detect beam deflection. The sensitivity of PDS is demonstrated by measuring small temperature rises in different media, and the technique is compared with thermal lensing (TL) and photoacoustic spectroscopy (PAS). PDS is found to be as sensitive as TL but offers greater versatility, especially for opaque or scattering samples, and in hostile environments. The paper also discusses noise sources and background considerations, highlighting the importance of laser pointing noise and electronic noise in PDS. Overall, the study provides a detailed analysis of PDS, showing its potential for high-sensitivity spectroscopy and its advantages over related techniques.This paper presents a comprehensive theoretical and experimental investigation of Photothermal Deflection Spectroscopy (PDS), a sensitive spectroscopy technique based on the photothermal deflection of a laser beam. The authors develop a general theoretical framework for both collinear and transverse PDS, considering both continuous wave (cw) and pulsed cases for solids, liquids, gases, and thin films. They derive expressions for the temperature distribution and optical beam propagation, and provide numerical evaluations to validate the theoretical predictions. The experimental setup includes configurations for transverse and collinear PDS, using various light sources and position sensors to detect beam deflection. The sensitivity of PDS is demonstrated by measuring small temperature rises in different media, and the technique is compared with thermal lensing (TL) and photoacoustic spectroscopy (PAS). PDS is found to be as sensitive as TL but offers greater versatility, especially for opaque or scattering samples, and in hostile environments. The paper also discusses noise sources and background considerations, highlighting the importance of laser pointing noise and electronic noise in PDS. Overall, the study provides a detailed analysis of PDS, showing its potential for high-sensitivity spectroscopy and its advantages over related techniques.