In this contribution of the SCOR working group 118 (New Technologies for Observing Marine Life), we review the state-of-the-art optical methods for obtaining information on phytoplankton species composition and taxonomic distribution in the ocean. Single-cell imaging systems are presented as well as methods for analyzing bulk optical properties to obtain information on the dominant species. We present methods based on both in-situ and laboratory measurements of optical properties, as well as from satellite remote sensing. The application of these methods to the specific condition of red tides (i.e. extreme blooms) is presented as an example. Present limitations and future development are discussed.