Assessing mobility in a thorough fashion is a crucial step toward more efficient mobile network design. Mobility has been centered around defining specific models aiming at specific human mobility kinds. However human mobility is not a unique and defined process. In the first part of this talk, I'll introduce a new approach aimed at rationalizing mobility expression, based upon mobility traits. In essence, it considers that a particular mobility of a given individual at a given time is the result of the influence of several traits (e.g. calendar following, social interaction, obstacle avoidance, map following...) instead of one all encompassing model. Moreover, recent research on mobility has been focused on two main points: analyzing models and studying their impact on data transport. These works investigate the consequences of mobility. In the second part of this talk, I'll focus on the causes of mobility. Starting from established research in sociology, I present SIMPS, a mobility model of human crowd motion.