Gelation describes the transformation from a liquid or fluid state to a somewhat solid state, well known from daily experience, making, for instance, a jelly pudding. Gels and gelation are studied quite extensively in chemistry and physics, and especially over the past three decades theoreticians have discussed the formation of gels and the relation between structure and properties quite extensively (percolation theory, fractals). In this chapter, we will first discuss the viscosity of solutions and how it changes during gelation. For an understanding of modern equipment, to analyse gelation, it is important to briefly discuss viscoelasticity andsimple models for a viscous fluids. We thendescribe how gelation is measured, from very simple methods to more elaborate ones. The chapter closes with a survey of theoretical models for gelation such as percolation, diffusion-limited cluster aggregation, mean field theory using the Smoluchowski equation, scaling analysis and polymerisation-induced phase transformation (PIPS). For all these models, predictions of gel time can be made, showing how the composition of the solution, the viscosity and the temperature affect it.