A wandering mind is an unhappy mind. Human beings often think about things not happening around them, past or future events, or even things that will never happen. This "stimulus-independent thought" or "mind wandering" is the brain's default mode of operation. While this ability helps people learn, reason, and plan, it may have emotional costs. Many philosophical and religious traditions suggest that happiness comes from living in the moment and resisting mind wandering. Laboratory experiments have studied the cognitive and neural bases of mind wandering, but little is known about its emotional consequences in everyday life. Experience sampling, which involves contacting people during their daily activities and asking them to report their thoughts, feelings, and actions, is the most reliable method for investigating real-world emotion. However, it is costly and time-consuming, limiting its use. To address this, the authors developed a web application for iPhones to collect real-time reports from a large sample of people across 83 countries. The database includes nearly 250,000 samples from about 5,000 people. The study analyzed data from 2,250 adults to determine how often people's minds wandered, what topics they wandered to, and how these wanderings affected their happiness. The results showed that people's minds wandered frequently, regardless of their activities. Mind wandering occurred in 46.9% of samples, with the highest frequency during most activities except making love. People were less happy when their minds were wandering than when they were not, and this was true during all activities. People were no happier when thinking about pleasant topics than about their current activity, but were considerably unhappy when thinking about neutral or unpleasant topics. The nature of people's activities explained a small portion of the variance in happiness, while mind wandering explained a larger portion. The variance explained by mind wandering was largely independent of the variance explained by the nature of activities, suggesting that they are independent influences on happiness. In conclusion, a wandering mind is an unhappy mind. The ability to think about what is not happening is a cognitive achievement that comes at an emotional cost.A wandering mind is an unhappy mind. Human beings often think about things not happening around them, past or future events, or even things that will never happen. This "stimulus-independent thought" or "mind wandering" is the brain's default mode of operation. While this ability helps people learn, reason, and plan, it may have emotional costs. Many philosophical and religious traditions suggest that happiness comes from living in the moment and resisting mind wandering. Laboratory experiments have studied the cognitive and neural bases of mind wandering, but little is known about its emotional consequences in everyday life. Experience sampling, which involves contacting people during their daily activities and asking them to report their thoughts, feelings, and actions, is the most reliable method for investigating real-world emotion. However, it is costly and time-consuming, limiting its use. To address this, the authors developed a web application for iPhones to collect real-time reports from a large sample of people across 83 countries. The database includes nearly 250,000 samples from about 5,000 people. The study analyzed data from 2,250 adults to determine how often people's minds wandered, what topics they wandered to, and how these wanderings affected their happiness. The results showed that people's minds wandered frequently, regardless of their activities. Mind wandering occurred in 46.9% of samples, with the highest frequency during most activities except making love. People were less happy when their minds were wandering than when they were not, and this was true during all activities. People were no happier when thinking about pleasant topics than about their current activity, but were considerably unhappy when thinking about neutral or unpleasant topics. The nature of people's activities explained a small portion of the variance in happiness, while mind wandering explained a larger portion. The variance explained by mind wandering was largely independent of the variance explained by the nature of activities, suggesting that they are independent influences on happiness. In conclusion, a wandering mind is an unhappy mind. The ability to think about what is not happening is a cognitive achievement that comes at an emotional cost.