The idea for this guide, Freshwater Algae in Northwest Washington, began in 2006 when the Institute for Watershed Studies (www.wwu.edu/iws) expanded its Northwest Lakes monitoring project to collect water quality samples from more than 70 local lakes. This volume, Cyanobacteria, is part of a series describing freshwater algae in Northwest Washington, and includes simple algal keys that can be used by students with little experience in taxonomy, as well as members of the general public. Although the emphasize is on freshwater lakes, samples collected in streams, seeps, waterfalls, and other lotic systems are included, with comments on whether the taxa are likely to be found in plankton samples.