Material extrusion or Fused Filament Fabrication (FFF) is a common additive manufacturing technology, where a filament is molten and deposited on the build surface according to a CAD-defined contour. After each layer the build platform moves one layer height down, so that the next layer can be deposited. A decent amount of studies has been investigated on the strength of the interlayer cohesion. However, nearly no study has dealt with the adhesion of this first deposited layer on the build platform, although it is a very critical step, as the print cannot be finished in a proper way, if the part detaches during the print. The present work aims at understanding and optimising the adhesion of different filaments (poly(lactic acid) and acrylonitrile butadiene styrene) on two commercially available build platform materials (glass and polyimide). The adhesion forces are measured by means of a self-constructed adhesion force testing device, in which printed strands are sheared-off and the maximum adhesion force is recorded, as a function of the build platform temperature. Additionally, contact angle measurements and investigations of the surfaces of the sheared-off strands complement the results. It was found that best adhesion is given, when the temperature of the build platform is slightly higher than the glass transition temperature of the deposited polymer. Furthermore, the investigations show that solely the surface tension and topography of the contact partners are not sufficient to fully describe the complex adhesion mechanism in FFF. Especially for the development of novel materials, the knowledge gained in this work is essential to improve the reliability or optimise the printing process.