While the cellular, and possibly the molecular, bases of some forms of learning appear to be at the threshold of understanding in some invertebrate preparations, the bases of learning and memory in complex nervous systems have proven considerably more complicated. However, increasing evidence points to synapse formation as a component of mammalian long term memory and related aspects of adult information storage. In early development, experience effects appear often to be encoded via the selective preservation of a subset of preformed connections. In adults, experience may both initiate synaptogenesis and select connections in the memory formation process.