In situ optical radiometric methods for the determination of seawater apparent optical properties are comprehensively addressed with the final objective of quantifying measurement uncertainties for remote sensing applications. Emphasis is placed on the presentation and assessment of calibration and measurement protocols in combination with schemes for the minimization of instrument, deployment and environmental perturbing effects. Specific investigations deal with the determination of the uncertainties produced by the non ideal cosine response of irradiance sensors in the normalization of the water-leaving radiance, the quantification of the immersion factors for both in-water radiance and irradiance sensors, the estimate of perturbing effects by deployment superstructures, self-shading and surface roughness for in{water measurements. Applications of in situ optical radiometric measurements, including the development of algorithms for the determination of the concentration or the apparent optical properties of seawater optically signi¯cant constituents from remote sensing data, and additionally the validation of primary remote sensing radiometric products, are also addressed and discussed in relation to the uncertainty of radiometric data.