Long-term trends in the Secchi depth of the Baltic Sea have been interpreted in terms of eutrophication (1, 2). The spectral attenuation coefficient K-d (490) can be estimated from remote sensing data (3). Given the empirical and theoretical relationships between diffuse attenuation and Secchi depth, it is therefore possible to estimate the trophic state from remote sensing data. This paper considers relationships among remotely sensed and in-water measured K-d (490), and Secchi depth data obtained during dedicated sea-truthing campaigns in the eastern Baltic Proper in 1999 (4) and in the western Baltic Proper/Himmerfjarden area during 2001 and 2002. In-water measurements are used to establish the relationship between the PAR and the spectral attenuation coefficient in the Baltic Sea via regression analysis. The analysis showed that in the area of investigation K-d(490) is about 1.48 times higher than K-d(PAR). This relationship is then used to define the link between the photic zone depth and the remote sensing optical depth, K-d (490)(-1). The results show that the depth of the euphotic zone is about 6.8 times K-d (490)(-1). The regression analysis between K-d (PAR) and Secchi depth confirmed previous work that K-d (PAR) is about 1.7 of the inverse Secchi depth. Furthermore, an in-water algorithm between Secchi depth and K-d (490) is used to simulate a Secchi depth map of the Bartic sea from seaWiFS K-d (490) data. This map is verified against sea-truthing data. K-d (490) data derived from satellite is compared to in situ Kd (490), and the sources of error are discussed.