This study examines the terrestrial and oceanic sources of atmospheric moisture contributing warm-season precipitation in the Mississippi River Basin. It is hypothesized that spatial patterns in the moisture supply may help to explain interannual variability in precipitation. Using back-trajectory analysis, we identify the most recent evaporative sources of precipitation falling in the basin. This spatial information is organized into time series of contributions from 19 selected source regions: eight oceanic (Temperate and Tropical North Atlantic, Gulf of Mexico, Caribbean, Temperate and Tropical North Pacific, Baja Pacific, Arctic), five Mississippi sub-basins (Missouri, Upper Mississippi, Ohio, Red/Arkansas, and Lower Mississippi), and six terrestrial regions external to the basin (Western, Southwest, and Southeast U.S.; Texas/Mexico; Northern North America; and Central/South America). Principal component analysis is applied to identify patterns in the space/time matrix of source area contributions. Covariability among the Missouri, Red/Arkansas, Texas/Mexico, Lower Mississippi, and Caribbean source regions is found to be an important factor in statistically explaining interannual variability in Mississippi Basin warm-season precipitation. A spatial pattern of surface pressure anomalies over central North America and the subtropical Atlantic and Pacific Oceans is temporally correlated with moisture transport across the southern boundary of the Mississippi Basin, suggesting a connection between basin warm-season precipitation and the atmospheric general circulation. As a complement to the back-trajectory tool, forward-trajectory analysis traces the rain-out destinations (sinks) of moisture evaporated from the basin, and helps to confirm previous findings on the importance of local terrestrial moisture supply (recycling) in Mississippi Basin precipitation.