The ability to manipulate neurons involved in memory formation and retrieval is a key factor to understanding neuronal circuits involved in cognition. In this chapter, we provide methods to tag and/or allocate neurons to a fear conditioning engram in the hippocampus and manipulate these neurons using optogenetic and chemogenetic approaches. For tagging, we describe IEG-based methods (TRAP2 mice or the viral RAM system) to express transgenes, such as designer receptors exclusively activated by designer drugs (DREADDs) or channelrhodopsin in fear engram cells. For allocation, the excitability of neurons in the CA1 region may be transiently and reversibly heightened using opsins, and heightened excitability increases the chance of the neurons being integrated into the fear conditioning engram. Allocated/tagged neurons then can be subsequently activated or silenced during the memory test using optogenetic or chemogenetic transgenes leading to heightened or decreased freezing, respectively.