Molecular motors are enzymes which convert chemical energy into mechanical work. Motor proteins such as kinesin, dynein or myosin are unidirectional stepping motors, which are involved in force generation and active intracellular transport. The properties of single molecular motors have been studied extensively both experimentally and theoretically in the last two decades. However, cargos are often transported by groups of cooperating molecular motors and much less is known about the properties of collective transport by motors. In this talk, we will discuss the collective transport of filament by motors (filament gliding assay) and focus on two subjects: i) filament stepping dynamics and investigate how single motor properties such as stalk elasticity and step size determine the presence or absence of fractional steps of the filaments. We show that each gliding assay can be characterized by a critical motor number, Nc. Because of thermal fluctuations, fractional filament steps are only detectable as long as N < Nc. The corresponding fractional filament step size is l/N where l is the step size of a single motor. ii)Several intracellular processes are governed by two different species of molecular motors, fast and slow ones, that both move in the same direction along the filaments but with different velocities. Here, we will discuss the transport of filaments by these motors. We find that a cooperative transport mechanism leads to three distinct motility regimes and to bistable switching between fast and slow transport. A surprisingly different behavior observed in experiments can be explained in a quantitative manner, using a theory based on single motor parameters.