Lecture 1: Above‐ et In‐Water Radiometry (Methods et Calibration Requirements) In situ optical radiometric measurements have direct application in the development and assessment of: theoretical models describing extinction of light in seawater; and empirical algorithms linking the seawater apparent optical properties to the optically significant constituents expressed through their inherent optical properties or concentrations. In addition, in situ radiometric data are essential for the vicarious calibration of space sensors and the validation of remote sensing products. The most accurate input data is always the most desirable for any bio‐optical modeling and calibration or validation activity. However, accuracy requirements impact methodological and instrumental investment which should be weighed against the specific need for each application. The lecture, after a brief introduction to radiometric concepts and terminology, addresses the fundamentals of above‐ and in‐water field radiometry. This includes a review of instruments, measurement methods and data analysis. Additional elements addressed in the lecture include an overview of calibration requirements and methods for ocean color field radiometry. These latter include an introduction to absolute radiometric calibration of radiance and irradiance sensors, determination of the geometric response of cosine collectors, characterization of immersion factors for in‐water sensors. Finally, inter‐comparisons of radiometric products derived from different measuring systems and methods are used to discuss individual performances. Bibliography G. Zibordi and K.J. Voss. Field Radiometry and Ocean Color Remote Sensing. In Oceanography from Space, revisited. V. Barale, J.F.R. Gower and L. Alberotanza Eds., Springer, Dordrecht, pp. 365‐398, 2010.