In semi-arid interior Alaska, precipitation is critical in regulating many important ecological and biogeochemical processes. For example, the growth of upland white spruce is strongly limited by growing season precipitation, and greenhouse trace gas fluxes are controlled in part by soil moisture. We have used a variety of tools at a network of sites in interior Alaska to determine variability in precipitation during the Holocene. On the millennial scale, data suggest a general increase in P-E during the Holocene. This trend is supported by lake level-reconstructions based on core transects at multiple sites, and by stable isotope analysis of cellulose, authigenic carbonates, and organic matter at several sites. The general increase in moisture balance is consistent with the sequence of major vegetation changes, such as the establishment of white spruce (ca. 8.5 ka BP; all ages in radiocarbon years), alder (ca. 6.5 ka BP), and black spruce (ca. 5 ka BP). Hydrologic models indicate that precipitation at 9 ka BP was 25 to 45% less than modern in the eastern interior. Data from several sites suggest relatively rapid increases in effective moisture at about 9-8.5 ka BP, and from about 4-3 ka BP. Records from sensitive closed basin sites suggest that near modern water balances were only established within the past 2 ka. Some deviations from this pattern occur, and may be due to regional climate variability, or to differences in the sensitivity of the hydrology at a particular site. Higher frequency variability is evident in proxy records from high sedimentation rate sites, and from tree ring analysis. In general, the data suggest significant decade-to-century scale changes in effective moisture, with evidence for both abrupt, and possibly cyclic variability (~ 100 yr). Several records show significant changes from the 19th to the 20th century. There is evidence in both lake-level and tree ring data that significant arid periods occurred during the latter stages of the little ice age.