This session on in‐water algorithms will trace the history of the development of algorithmic methods applied to Ocean Colour data starting with the very first algorithms applied to global datasets obtained from the CZCS sensor. We will outline the evolution of algorithm development as our knowledge (and data!) on the optical properties of both open‐ocean and coastal waters have improved over the last three decades. Additionally we will examine the mathematical and statistical approaches (neural networks, non‐linear optimisation, spectral un‐mixing, principal component analysis etc.) that have been explored to make best use in using the radiometric quantities measured by the sensors in retrieving the relevant geophysical quantities of interest. Specific attention will be placed on emphasising the complexities of applying such methods in coastal regions, and considerations will be made on the limitations and uncertainties that need to be understood. Furthermore we will analyse the parallel progress of both the empirical and semianalytical method, and consider the merits and deficiencies of each of these, providing a clear understanding of the difference between these methods and their practical application in the operational processing of data (see flowchart below). Complimentary to this we will specifically consider the results from an intensive round robin intercomparison of different semi‐analytical methods (performed by the IOCCG). In considering all of these various aspects of different available algorithms we will underline which algorithms have been considered for routine processing by the major space agencies (and why). Continuing we shall address the relative benefit of using standard global coverage products compared to regional algorithms and vice versa, and explore various alternatives for the implementation of regional algorithms. Here we will investigate the “minimum requirements” for the implementation of such regional algorithms (i.e. required datasets, “Level” of satellite data required, computing requirements). Finally we will make some considerations on the future direction for research on these topics. And deal with any real world examples/questions that participants may have and want to address. Bibliography In preparation for the course, it is suggested that participants consult the following, freely available, IOCCG reports (a more detailed bibliography, on specific topics, will be provided during the course): Working group on Ocean Colour in Case 2 Waters (Chaired by Shubha Sathyendranath): IOCCG Report 3 (2000). Remote Sensing of Ocean Colour in Coastal, and Other Optically‐Complex, Waters (http://www.ioccg.org/reports/report3.pdf) Working group on Ocean Colour Algorithms (Chaired by ZhongPing Lee): IOCCG Report 5 (2006). Remote Sensing of Inherent Optical Properties: Fundamentals, Tests of Algorithms, and Applications. (http://www.ioccg.org/reports/report5.pdf)