This thesis describes the reproductive biology of the endemic Australian gekkonid genera Nephrurus and Underwoodisaurus. These genera form an important group for investigation because they have both arid and mesic species and occupy both tropical and temperate climates, enabling a comparison of features related to their habitats. Analysis of museum records shows that Nephrurus species largely occupy central and northern Australia while Underwoodisaurus species largely occupy southern Australia. A morphometric and meristic analysis was carried out on over 1000 alcoholic specimens and over 200 live specimens using up to approximately 50 characters (depending on species, gender and status) of all eleven species (N. amyae, N. asper, N. deleani, N. laevissimus, N. levis, N. sheai, N. stellatus, N. vertebralis, N. wheeleri, U. milii and U. sphyrurus). The distinctiveness of the genera Nephrurus and Underwoodisaurus is confirmed. The distinctiveness of a primitive and a derived group within Nephrurus is also confirmed based on radiographic and morphometric data. Nephrurus deleani is allocated to the primitive group (includes N. laevissimus, N. levis, N. stellatus and N. vertebralis). Also, N. amyae and N. sheai are allocated to the derived group (includes N. asper and N. wheeleri) based on phalangeal formula and lack of autotomy plane in the tail. Numerous sexual dimorphisms were demonstrated, including smaller males than females in all species (except u. milii), relatively longer limbs in males in most species, relatively longer tails in males and more subdigitallamellae in male U. milii and U. sphyrurus compared to females. A distinctive ventral escutcheon of enlarged scales occurs in adult Underwoodisaurus males. Scattered and distinctive preano-inguinofemoral tubercles are found only in the adult males of N. deleani, N. laevissimus, N. levis and N. stellatus. Mating (involving a primitive grip by the male) and egg-laying (involving nesting burrow excavation) behaviours are described in detail and are typical of lizards in general. Much of the egg data support the hypothesis that Nephrurus eggs are better adapted to aridity than are Underwoodisaurus eggs. Egg size relative to maternal size is significantly greater among diplodactyline and eublepharine geckos compared to gekkonine and sphaerodactyline geckos. Relative egg size is also greater among arid Nephrurus and Diplodactylus species compared to mesic Underwoodisaurus and Diplodactylus species. Arid Nephrurus species have a significantly greater relative clutch mass compared to mesic diplodactyline species and compared to Underwoodisaurus species. Bicone values were determined for N. deleani. N. levis and U. milii and U. sphyrurus egg shape, but values were variable and therefore not as significant in gecko compared to avian eggs. The mean increase in mass during incubation of Nephrurus eggs is 24.7 ± 26.4 % of initial mass, which is significantly less than the mean increase for U. milii of 42.1 ± 27.1 %. Transmission and scanning electron microscopy and energy dispersive spectrometry of Nephrurus and of Underwoodisaurus eggshells showed that calcium bicarbonate was distributed largely in the outer region of the shell. Extraembryonic egg contents and egg residues after hatching contain none or insignificant amounts of reducing sugars. Fresh laid Nephrurus levis eggs contain approximately 80 % water. After desiccation they contain approximately 53 % protein, 33 % lipid and 7 % ash, with an energy density of approximately 22.2 kJ/g. The eggs of U. milii were not significantly different from those of N. levis in composition. Nephrurus levis and U. milii embryos both have an exponential pattern of oxygen consumption during development. The mean energetic cost of embryonic development is 5.82 ± 0.83 kJ/g dry mass for U. milii at 2SOC and 5.49 ± 0.79 kJ/g at 30°C. This compares with 8.64 ± 2.2 kJ/g dry mass for N. levis at 25°C and 8.57 ± 3.4 kJ/g at 30°C. There was no significant difference in the effects of two substrate water potentials (-100 and -450 kPa) on incubation duration at both 25°C and at 30°C. The Q,o for duration of incubation of U. milii eggs at 25°C and 30°C was 2.3. Limited egg retention was found in N. levis. Measurements of adult testes and ovarian follicles of all Nephrurus and Underwoodisaurus species indicated large variation in size, even within the breeding season, indicating apparent low synchrony of male and female reproductive systems. Mean testis size increases with snout-vent length throughout life. Histology of testes showed spermatogenesis in all seasons of the year, but probably not mid-winter in southerly distributed species. Sperm storage occurs in Nephrurus females, and possibly in Underwoodisaurus species. Tails of Nephrurus species are multi-functional, being used in alarm responses, feeding, courtship and mating. No territoriality was demonstrated by Nephrurus species or by U. sphyrurus but adult male U. milii (probably the most abundant of all eleven study species) demonstrated strong territorial behaviour particularly in the breeding season.