The modeling of dynamic activity generation is high on the research agenda in activity-based transport demand modeling. The concept of dynamic needs has been put forward as such a mechanism. Needs that underlie the generation of such discretionary activities as social, recreational, and sports activities is investigated. Three surveys were conducted to elicit, establish, and analyze the needs. Qualitative face-to-face interviews were carried out with a laddering technique to reveal need dimensions through an exhaustive classification of discretionary activities. Quantitative approaches were then used to determine which needs are equivalent in their effects on activities and, hence, can be merged. Finally, a questionnaire-based survey involving a large sample of individuals was used to measure personal levels of the needs identified and to correlate these measures with socioeconomic and behavioral characteristics. Six independent needs emerged from this research: physical exercise, social contact, relaxation, fresh air and outdoors, new experiences, and entertainment. Many-to-many relationships between activities and needs support the hypothesis that substitution relationships may play a significant role in activity generation. This observation implies that current practice in activity-based modeling of focusing on activities may produce biased results in development of dynamic models of transport demand. Furthermore, the results show that personal levels of these needs correlate with various socioeconomic as well as behavioral variables.