This chapter examines one of nine critical forces behind purchase decisions that make mobile advertising so powerful: trajectory. An individual's trajectory is the physical, behavioral trace of his or her offline movements. Firms can measure when we walk past their physical stores, when we come through the front door, when we walked up to the second floor, and so on. Trajectory has three dimensions: time, route, and velocity. Time includes the starting and ending point of the trajectory and the day of the week. Route is not location itself, but rather a way to determine how similar someone's spatial trajectory is to others. Velocity contains information about how fast the individuals are moving. Underlying these three dimensions is a fourth and far more granular dimension called semantics. Semantics takes a number of factors into account, such as the likelihood that someone may visit a certain store, how much time they spend there, how much time they spend moving to another location, and how related or unrelated those two stores are.