The word desert has no fully agreed upon meaning among geographers. Many prefer, in fact, the terms dry lands or arid lands. Part of the problem in arriving at a deﬁnition that will satisfy everyone is that many popular images of North American deserts—bleak expanses of sand, lizards seeking shaded cover from a broiling sun, saguaro cactus— don’t begin to ﬁt all of North America’s deserts. Saguaros grow only in the lower Sonoran Desert; no lizards inhabit the polar deserts of Alaska; and much of the Great Basin Desert is covered with brush and grasses, even open stands of piñón pine and juniper. To biologists, deserts are dry habitats that accommodate many fewer species than other habitats such as forest land or tall grass prairies. Desert habitat is further characterized by plants and animals with unique strategies for ﬁnding and conserving water. And they are set apart from other lands by low humidity and scant rainfall, and by sometimes striking ﬂuctuations in daily and seasonal temperatures. For nonbiologists, what makes deserts distinctive is not ecosystems but landforms—stony plains of desert pavement, arroyos cut by ﬂash ﬂoods, dune ﬁelds. Excluding the polar deserts of Alaska and the arid, volcanic landscapes of Hawai‘i, there are four deserts in the United States. The “warm” Chihuahuan and lower Sonoran Deserts of California, Arizona, New Mexico, and Texas; and the “cold” deserts—the upper Sonoran of Arizona, New Mexico, and Utah; the Mojave of California and Nevada; and the Great Basin Desert, stretching all the way to Canada and taking in parts of California, Nevada, Utah, Colorado, Idaho, Oregon, and Washington. The most arid of them all is the Mojave. In the “high desert” of eastern Oregon and Washington, desertiﬁcation caused by overgrazing has converted large stretches of dry grassland into landscapes dominated by sage and scrub vegetation and by such exotic plants as Russian thistle (tumbleweed) and cheat grass. Scientists are still discovering new forms of life in the desert, contradicting the persistent notion that deserts are barren, lifeless, worthless.