The North American Monsoon (NAM), which typically occurs from the beginning of July through the middle of September, is an important source of precipitation for the southwestern United States and northwestern Mexico. In some locations, monsoon precipitation accounts for up to 50% of annual precipitation. The purpose of this study is to document the NAM climatology (specifically, the dates of onset and demise, monsoon length, and amount of precipitation) and to examine the nature and causes, both local and remote, of interannual and decadal variability in NAM characteristics. The methodology of Ellis et al. (2004) was employed to examine the dates of onset and demise, monsoon length, and amount of precipitation for the NAM in the southwestern United States between 1918 and 2006. On average the NAM begins on June 28th and ends on September 14th (an average season lasts 78 days). The shortest NAM season was only 18 days (1920) and the longest monsoon season was 118 days (2003). During the NAM season, the NAM region in the southwestern United States receives, on average, 138 mm of precipitation. However there is significant interannual variability in the amount of monsoon precipitation (standard deviation = 27 mm). The wettest year (1999) received 218 mm of precipitation, while the driest year (1920) received only 42 mm. In addition to the significant interannual and decadal variability in NAM characteristics there are also statistical significant secular trends. The NAM has been starting earlier, ending later, and NAM precipitation has increased slightly between 1918 and 2006. Both local (e.g., soil moisture and snow water equivalent) and remote forcings (e.g., ENSO, PDO, AMO, NAO) are associated with the observed variability and trends in NAM characteristics.