The relationships between climatic factors and fluctuations in dated tree-ring widths are statistically evaluated. A wide ring indicates that the year's climate was moist and cool, and a narrow ring dry and warm. In general, ring width relates to a 14-month period from June through July but most tree-ring chronologies exhibit a closer relationship with autumn, winter, and spring moisture than with summer moisture. The climatic relationships for evergreen trees are attributed largely to the influence of environmental factors on photosynthesis and the accumulation of food reserves. Under abnormally dry and warm conditions, especially during the autumn, winter, and spring, little food is accumulated, new cells are formed more slowly during the growing period, and the resulting ring is narrow. Relative 10-yr. departures are calculated for the entire length of 26 tree-ring chronologies from western North America. Those portions after 1500 are used to map areas of high and low moisture. Periods of widespread drought are noted in 1576–1590, 1626–1635, 1776–1785, 1841–1850, 1871–1880, 1931–1940. Periods of widespread and above average moisture occurred during 1611–1625, 1641–1650, 1741–1755, 1826–1840, 1906–1920. The moist periods of 1611–1625, and 1906–1920 were most widespread and markedly above average.