We will start by determining the molecular composition of the synapse, and its changes during synaptic activity. We will determine the locations of synaptic organelles and proteins, their copy numbers, their post-translational modifications, and their interactions. We will focus on two classical models, hippocampal cultures and synaptosomes, which are ideal for imaging and biochemical studies, respectively. Together with analyses of synaptic lipids and RNAs, these parameters will serve to generate a structural model of an averaged synapse at rest, during activity, and after activity. We will complement this model with several key functional parameters, ranging from synaptic protein mobility to the principles behind the formation of synaptic active zones. The structural and functional parameters will be integrated in modeling studies aiming to explain and predict synaptic function and plasticity. At the same time, we will initialize analyses of a limited number of synaptic disease models, to decipher the quantitative differences that separate the pathologically altered synapses from normal ones. We will thereby test the hypothesis that specific synaptic diseases are caused by differences in the numbers, activity and/or organization of defined synaptic elements.