Determining what influences mood is important for theories of emotion and research on subjective well-being. We consider three sets of factors: activities in which people are engaged; individual differences; and incidental variables that capture when mood is measured, e.g., time-of-day. These three factors were investigated simultaneously in a study involving 168 part-time students who each responded 30 times in an experience sampling study conducted over 10 working days. Respondents assessed mood on a simple bipolar scale - from 1 (very negative) to 10 (very positive). Activities had significant effects but, with the possible exception of variability in the expression of mood, no systematic individual differences were detected. Diurnal effects, similar to those already reported in the literature, were found as was an overall "Friday effect." However, these effects were small. Lastly, the weather had little or no influence. We conclude that simple measures of overall mood are not greatly affected by incidental variables.