Ultracold neutral atoms can be trapped and coherently manipulated close to a surface using chip-based magnetic microtraps. This opens the possibility of studying interactions between atoms and on-chip solid-state systems such as micro- and nanostructured mechanical oscillators. This thesis reports experiments, where a controlled coupling between a Bose-Einstein condensate and a micromechanical oscillator is realized for the first time. The interaction relies on surface forces experienced by the atoms trapped at about 1 micrometer distance from the mechanical structure. The surface forces are...