In the adult somatosensory cortex the majority of the spines are stable. However some spines appear and disappear over time. EM analysis showed that many newly formed spines do not bear a morphologically defined synapse. In this thesis I describe studies that we have conducted to directly probe the dynamics of excitatory and inhibitory synapses in vivo. We used in utero electroporation of GFP-tagged PSD-95 to investigate the relationship between spine and PSD stability in neocortical L2/3 pyramidal cells in vivo. We then adapted a single cell electroporation method for chronic imaging of synaptic proteins. The technique has a high spatial and cell type selectivity, a low background fluorescence and does not affect developmental processes. As a proof of principle we transfected various synaptic proteins, some of which where thus far only successfully studied in vitro.