There have been many theoretical descriptions of the nucleation process but very few detailed experimental research has been carried out. The nucleation process was systematically analysed with the aim of gaining a deeper understanding of foam formation and the nucleation process, in order to develop possibilities in influencing it and to ultimately reduce the thermal conductivity. A method was developed whereby the nucleation process could be observed in-situ and subsequently analysed by means of a PC-controlled camera attached to a stereo microscope. The increase in volume due to foam formation and its exothermic reactivity was taken into consideration. Thereby, the amount of cell nuclei in the initial phase was examinable and cell growth including coalescence could be followed. The initial nuclei compared favourably to the final number of cells and to their size, providing a consistency with real foaming conditions. Detailed research was carried out on the effect of various types and amounts of blowing agents (e.g. carbon dioxide, cyclopentane, perfluoroalkanes), surfactants, catalysts, fillers and isocyanates on the nucleation process. Other important factors influencing the nucleation process are the component's viscosity and surface tension and their relationship with each other during the foaming process. The surface tensions and viscosities of the various systems used were also investigated using several known methods. Perfluorohexane-blown foams have larger nucleation numbers than either CO2- or cyclopentane-blown foams. This is only true when an emulsifier is present. Contrary to the literature, the initial number of nuclei did not vary with respect to surfactant type. Nucleation numbers proved to be independent of surface tensions values proving that lowering the surface tension does not automatically ensure higher nucleation numbers. Surfactants were shown to have either stabilising or emulsifying abilities. An improvement in nucleation numbers was obtained by improving the compatibility of the A- and B-components through the use of prepolymers. One can conclude th at nucleation is a complex heterogeneous process in which surfactants, catalysts and fillers play a minor roll.