Memory is a phenomenon of defined neurobiological substrate, enabling undertaking decisions basing on past experience. Owing to its adaptive value it appeared early in the evolution. Memory consists in creating of engrams – the representations of environmental events, and their storage, recall and processing. It is formed in three stages of different nature and duration of the engram: sensory memory (1 s), working memory (approximately 30 s) and durable memory, subdivided into rapidly formed, rather labile short-term memory (up to few hours) and strongly consolidated long-term memory (years and decades). Representations of various type are, depending on their kind, stored either as prone to amnesia explicit declarative memory (subdivided into semantic, episodic and autobiographic), or as implicit, difficult to verbalization, amnesia-resistant procedural memory. The nature of engrams of various types of memory differs. The engrams of long-term memory are stored in changes in the neuronal network, induced by formation of new proteins after activation of expression of various genes resulting from simultaneous stimulation of neurons by glutamate and a monoamine neurotransmitter, leading to phosphorylation and activation of promoters controlling genome. The engrams are localized in the cortex but are sufficiently dispersed to survive the loss of a part on neurons in the network. In the mammalian brain there exist three complementary memory systems, centered on the hippocampus, striatum and amygdala, and involved respectively in spatial and declarative, motor and emotional memory. Studies on animal models have revealed molecular and biochemical aspects of engram formation. On this base the pharmacotherapy of pathological cognitive deficits and methods for enhancement of normal memory may be proposed.