The peninsular-side islands in the Midriff region of the Gulf of California are sometimes referred to as the San Lorenzo chain. Until recently, the archaeology of these islands was completely unknown. Basic exploration conducted since 2004 has shown that nearly every major island in this archipelago was exploited by indigenous peoples, some intensively. Although the resources of each island, and hence the archaeological assemblages, are distinctive, there are many elements in common. Sites include camps, quarry-workshops, and feature-rich localities of unknown function. Most artifacts are simple flaked stone tools that emphasize useful working edges rather than overall form. Milling stones are unshaped but some are well-used. Features include stone circles, piled rock enclosures, talus pits, rock clusters, and rock cairns. In general, the archaeological record appears to be a simplified version of the Comondú culture of the adjacent peninsular coast. However, one site is a historic Seri camp, and linguistic and historical evidence suggests that these people may have travelled widely among the islands. Regional chronology suggests that native peoples could have exploited the islands over several millennia, raising the possibility that they might have had a significant role in shaping modern insular ecosystems. Although we need more information on the prehistory of the archipelago, one island, Rasa, also offers a superb opportunity for historic archaeological investigations.