Spontaneous thoughts occur by default in the interstices between directed, generally taskoriented thoughts or moments of perceptual scrutiny. Their contents are overwhelmingly related to thinkers' current goals, either directly or indirectly via associative networks, including to past and tentative future goals. They are triggered by ambient goal-related stimuli or segments of own thought. Most spontaneous thought segments are very brief, a matter of seconds, but may be much longer. Their typical brevity generally permits wide variation in content, reflecting the individual's current agenda of goals. Their evocation is accompanied by emotional responses that vary widely in type, valence, and intensity. These properties of the emotional accompaniments probably determine which content area receives priority for thought. Given these properties of thought flow, spontaneous thoughts are highly adaptive as (1) reminders of the individual's larger agenda of goals while occupied with pursuing any one of them, (2) promotion of planning for future goal pursuits, (3) review and deeper understanding of past goalrelated experiences, and (4) development of creative solutions to problems in goal pursuit. In the face of threats to successful pursuit of important goals or the prospect of failure, the same 2 mechanisms may occasion repetitive but unproductive thoughts about the pursuit, the consequences of the failure, or the self, and strong negative emotions steering the train of thought may lead to narrowing of its focus, thus producing rumination. The disposition to ruminate is in part a function of states of negative affect and individual differences such as strength of focus on particular thought themes, momentum of particular emotional states, and neuroticism. Rumination is a common occurrence during low mood or outright depression that accompanies the process of disengaging from goal pursuits that have become unrealistic or overly expensive. The typically reduced interest in alternative goals during this phase further limits the range of thought content and the individual's resources for re-engagement with gratifying goal pursuits. For assessment and clinical purposes, spontaneous thought content can potentially provide information not captured by other procedures. When an individual's spontaneous thoughts are themselves problematic, empirically validated methods for improving the individual's motivational structure provide a likely best treatment strategy.[354 words]