This chapter introduces the fundamental concepts of neuromodulation within the nervous system, focusing on the intricate roles of neurotransmitters, synaptic plasticity, and neural circuits in modulating neural functions. It elucidates the mechanisms by which neurotransmitters influence neuronal excitability and interaction and explores how these chemical messengers contribute to the dynamic regulation of neural circuits. Synaptic plasticity is discussed as a critical process for learning and memory, facilitated by modifications in synaptic strength through mechanisms such as long-term potentiation (LTP) and long-term depression (LTD). This chapter sets the stage for understanding how these biological processes underpin not only basic neural activity but also complex behaviors and cognitive functions. The insights gained underscore the significance of neuromodulation in neuroscience research and its potential therapeutic applications for neurological and psychiatric conditions. As the foundational entry into a series on brain stimulation techniques, this discussion prepares the reader for deeper explorations into how these neuroscientific principles can be applied in clinical and therapeutic settings.