Comparatively few insects live in marine habitats, despite the fact that the oceans cover 71% of our planet’s surface. Part of the reason may be that insects evolved on land, yet freshwater forms have been in existence for at least 245 million years. After summarising insect groups known to be associated with saline habitats, this paper examines the evidence both supporting and refuting the three main theories on this anomaly. These theories centre around physical barriers, chemical barriers, and pre-existing niche occupancy by crustaceans. Many marine insects have become adapted to overcome physical barriers such as waves, tides, and currents, and similarly have overcome the physiological and osmoregulatory problems associated with living in salt water. In large freshwater lakes, some insects have achieved complete independence from the shore, including a planktonic stage, allowing them to permanently inhabit open water; yet most marine insects are to be found only in “bridging” marine habitats (estuaries, saltmarshes, mangrove swamps, and the intertidal zone). Pre-emption by crustaceans has some merit, but it seems highly unlikely that, after millions of years of simultaneous existence, insects, with their formidable competitive powers, should still be excluded from most marine niches. Whereas many insect species possess some of the necessary adaptations for access to the sea, relatively few seem to possess all of the necessary adaptations to allow them to be fully marine.