The nature and distribution of the synaptic changes that underlie memory are not well understood. Here we examine the synaptic plasticity behind context fear conditioning in male and female mice and find that new learning produces synaptic potentiation specifically onto engram neurons in the basolateral amygdala. This potentiation lasts at least 7 days, is reversed by extinction, and its disruption impairs memory recall. High frequency optogenetic stimulation of the CS and US-activated ensembles, or biochemical induction of synaptic potentiation in US-responsive neurons alone, is sufficient to produce a context fear association without prior associative training. These results suggest that plasticity of CS inputs onto US-responsive amygdala neurons underlies memory formation and is necessary and sufficient to establish context fear associations.