Nanoindentation has become the primary method for quantifying the mechanical properties of thin film materials. However, a mechanistic understanding of the phenomena occurring during nanoindentation of thin films is still lacking. We report results from our development of a new technique, that of in-situ nanoindentation within the transmission electron microscope. With this technique, we can observe in real time and at high spatial resolution deformation behavior and, in most instances, correlate this with quantitative load displacement characteristics of the indentation. In our experiments, a relatively sharp diamond is positioned at the edge of an electron transparent thin film using a combination of mechanical and piezoceramic actuation. The actual indentation is performed in a voltage-controlled manner, through piezoactuation alone. In this presentation, we will detail the necessary steps required to obtain accurate, reproducible quantitative information, with particular attention paid to issues related to load frame compliance and the actuation characteristics of the piezoceramic. Results from the indentation of aluminum films on silicon will demonstrate the utility of the technique. We find that in aluminum the onset of plastic deformation is accompanied by the sequential nucleation and propagation of geometrically necessary prismatic dislocation loops into the material, which act to accommodate the volume of the diamond within the material bulk. We will present additional results on the the effect of grain size and proximity of the indenter to grain boundaries on dislocation behavior.