The Tutorial presents the theoretical aspects of photothermal beam deflection spectrometry (BDS) in skimming configuration based on the complex ray theory. After a brief introduction on photothermal spectroscopy, an abridged review of the literature highlighting the theoretical building blocks and its diversified applications is given. The complex ray theory-based 1D and 2D models for evaluating thermophysical, optical, transport, and structural parameters are discussed in detail. The experimental setup, optimization, and applications of the BDS are further elaborated upon.