Traffic demand, or demand for short, describes when and where travelers want to travel. It can specify a number of choices, such as destination choice, departure time choice, mode choice and route choice. Which choices are specified in the demand depends on the application. Demand results in trips which manifest in moving GTUs over the network. GTUs are placed on the network by a vehicle generator, or GTU generator. These generators are the interface between demand, and the traffic simulation. This chapter describes two ways in which demand can be defined in OTS, an origin-destination matrix, or by using split fractions. Next, the vehicle generation methodology is explained, including some of the major components for vehicle generation. The last section of this chapter discusses how vehicle generators are created based on demand. Throughout this chapter, GTU generator is interchangeably used for vehicle generator.