Our long term goals are to develop a better understanding of the relationships between upper ocean optical properties and particulate and dissolved seawater constituents, and to determine how these relationships are influenced by physical processes. Specific long term objectives include both predicting and modeling optical variability relevant for biological processes, such as phytoplankton photosynthesis, and retrieval of information about the biomass and activity of plankton from optical measurements. The approach we have taken employs techniques for characterizing and assessing the optical properties of particles, using both in situ and ship board instrumentation and both bulk and single particle methods. Our primary tools are flow cytometery for assessing individual particle light scattering and fluorescence properties, spectrophotometry for measuring bulk dissolved and particulate absorption spectra (including separation of phytoplankton pigment absorption from the bulk aborption via methanol extraction), and spectral underwater radiometry. Our goal is to conduct flow cytometric and spectrophotometric measurements both on discrete water samples and with in situ instruments. In situ measurement provides the opportunity for relatively unperturbed sampling, with generally greater spatial resolution, while analysis of discrete water samples continues to allow more detailed characterization of optically active seawater constituents. We have employed our sampling methods during the Coastal Mixing Optics field study in continental shelf waters south of Cape Cod.