Lecture 2: In Situ Radiometric Products (uncertainty analysis and applications) Data products from in–water radiometric measurements generally include spectral values of: irradiance reflectance, remote sensing reflectance, normalized water–leaving radiance, diffuse attenuation coefficient and the so called Q‐factor. Data products from above–water radiometric measurements are generally restricted to the normalized water–leaving radiance and the remote sensing reflectance. By restricting the analysis to the normalized water‐leaving radiance, the lecture addresses the various sources of uncertainties affecting in situ radiometric measurements (e.g., accuracy of absolute calibration, superstructure perturbations, changes in illumination conditions, wave effects and selfshading for in‐water methods only). Emphasis is placed in the evaluation of methods allowing for the minimization of the various perturbing effects and additionally in the quantification of contributions of these latter to uncertainty budgets. Further element considered in the lecture is the application of in situ radiometric data to the assessment of satellite primary products (i.e., the normalized water leaving radiance determined from top‐of‐atmosphere radiance corrected for the atmospheric perturbations). Focus is placed on the use of in situ data to evaluate differences in cross‐mission products (i.e., normalized water leaving radiance from SeaWiFS, MODIS‐A, MODIS‐T and MERIS), variations in space system performance with time and intra‐annual changes in accuracy. 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.