The cause of FM remains unknown, and more than half of all FM patients do not experience adequate pain relief from current treatment. Identifying the mechanisms of unexplained pain in people with FM is necessary to develop more beneficial treatments. One possible cause of FM may be associated with problems in how the body processes pain. People with FM appear to be hypersensitive to stimuli that normally are not painful. Specifically, the brain and spinal cord may not signal sensations correctly, resulting in abnormal pain sensations. This study will use functional magnetic resonance imaging (fMRI) to examine how pain is processed in the brain of adults with FM compared with adults with rheumatoid arthritis (RA), another chronic disease characterized by long-term pain with a known cause. This study will also determine the effect that anticipation and attention have on the processing of nonpainful stimuli in women with FM and RA. Both groups of people will also be compared to a control group of healthy people.