Although gels are ubiquitous in fundamental science, technological applications and also in our daily life, their structural and dynamical properties are not well understood. In contrast to other systems that show a slow relaxation, such as glass-forming liquids, the structure of gels is given by an open network that is thought to be responsible for the unusual dynamical properties of these systems. I will discuss this issue by means of different models and recent numerical results. In particular, I will show that the strong length scale dependence of the dynamics in gel forming systems turns out to be tightly related to the formation of the gel structure. The mesh-size of the incipient gel network may correspond to a crossover length between dramatically different relaxation processes, from stretched to compressed exponentials. These features have no analogue in dense glassy systems. Finally, when the volume fraction is increased in the gelation regime, the structural relaxation begins to be affected also by the crowding of the particles, and a crossover to the glassy regime can be found.