Pore formation in keyhole Nd:YAG-laser spot welding has been investigated experimentally and theoretically. High speed X-ray imaging of the process delivers the shape of the keyhole just before switching off the laser beam. Starting from this shape the keyhole collapse is calculated by the aid of a mathematical model, distinguishing various physical phases of differing time scales, i.e. post-vaporisation, cooling, relaxation flushing of the excess vapour, and surface tension driven collapse of the keyhole along with vapour and melt flow. In case of a bottleneck-shaped keyhole, which is likely to form due to the paradox of flow inversion, necking forms a cavity, thus trapping vapour. According to X-ray observation and numerical simulation a contracting bubble forms that rises and is finally trapped by re-solidification of the melt, thus forming a pore. Different strategies for the prevention of pores are discussed, namely pulse-tail shaping for controlled keyhole contraction, welding under low ambient pressure which rarefies the keyhole vapour, and proper choice of the process parameters like spot size, pulse duration and shielding gas pressure for avoiding a bottleneck-shaped keyhole