Biological invasions into five nature reserves in arid environments are examined. Vascular plant invasions have not been particularly severe in the four continental reserves except along watercourses, where introduced phreatophytes have established extensively, crowding out native vegetation and often lowering water tables. The invasive genus Tamarix affects dozens of reserves in southwestern USA and would appear to be an outstanding candidate for biological control. Feral ungulate invasions have negatively affected most reserves, but are generally controllable, given the resources and the will to accomplish the task. Alien fish, often casually introduced by sportsmen or aquarists, are a major threat to native fish of arid land reserves. The arid insular reserve studied, though located less than 70 km offshore, has more extensive invasions than the continental reserves. Removal of introduced herbivorous mammals from it is allowing recovery of native shrubs at the expense of invasive herbaceous species.