This guide presents species of marine mollusks from Southwest Florida, focusing on the islands of Sanibel and Captiva, and extending farther south to the Ten Thousand Islands and north to Tampa Bay. The first version of this guide was posted online in 1998. Most specimens illustrated (but not all) reside in the collection of the Bailey-Matthews National Shell Museum, and all specimens originated from the study area. The guide is augmented and updated on a regular basis, so please make sure to check back here often. I am indebted to many individuals who consistently supported this effort by providing materials, facts, photos, editorial advice, and suggestions over more than twenty years. I am particularly indebted to Rüdiger Bieler, Lorin Buckner, Lois Dunnam, Jorden Falk, Phil Fallon, Emilio F. García, Leonard Geyer, John Greenlaw, Greg Herbert, Kelsey Hausmann, Susan J. Hewitt, Marcus Huber, Marlo Krisberg, Harry G. Lee, Barry McBroom, Rebecca Mensch, Paula M. Mikkelsen, Jake Mitchell, Kim Nealon, Bruce Neville, Ann Palmer, Harold Payson, Ken and Alice Piech, Hal Pilcher, Michael Reardon, Steve Rosenthal, James Scatterday, Bruce Schulz, Phyllis Sharp, Linda Shockley, Amy Tripp, and Ángel Valdés. Despite their generous assistance to this project, any shortcomings are my own. This resource was supported in part by grants IMLS MA 05-12-0450-12, IMLS MA 30-15-0276-15 (both from Institute of Museum and Library Services), and NSF TCN 2001528 (National Science Foundation.) Happy Shelling, José H. Leal, Ph.D. Science Director & Curator