Phytochrome photoreceptors integrate red and far-red light to inform vital adaptations of physiology, development, and lifestyle in manifold plants, bacteria, and fungi. Once covalently assembled with bilin chromophores, phytochromes traverse their metastable Pr and Pfr states depending on light quality and intensity. This chapter provides protocols for the analyses of phytochromes by absorption spectroscopy to determine their molar extinction coefficients, their photostationary state assumed under constant illumination, and their quantum yields for the light-driven Pr ⇄ Pfr interconversion. Knowledge of these parameters pertains to the natural role of phytochromes and their deployment in optogenetics and biotechnology.