Trained immunity is an academic concept and research field investigating the ability of the innate immune system to develop long-term memory after initial exposure to certain stimuli, leading to an enhanced, non-specific response to subsequent infections or challenges. This concept explores the epigenetic, metabolic, and functional reprogramming of innate immune cells, such as monocytes, macrophages, and NK cells, which results in a heightened state of readiness and altered cytokine production upon re-stimulation. Its significance lies in expanding the understanding of immune memory beyond the adaptive immune system and its potential for developing new prophylactic and therapeutic strategies targeting innate immune responses.