The chapter discusses the energy demands and supply mechanisms of synapses in the brain. It highlights that most brain energy is used on synaptic transmission, emphasizing the importance of understanding how energy is provided and utilized by synapses. The authors review the energy consumption of various subcellular processes, such as ion pumps and neurotransmitter release, and estimate that synaptic transmission consumes a significant portion of the brain's energy. They also explore how pre- and postsynaptic terminals are optimized to maximize information transmission while minimizing energy use, and how ATP provision to synapses is regulated to meet their energetic needs. The chapter further examines how synaptic energy use changes with development, synaptic plasticity, and between wake and sleep states, and discusses the consequences of disrupted energy supply to synapses, including neuropathology. Additionally, it delves into the role of mitochondria in providing energy to synapses and the mechanisms by which astrocytes may contribute to the energy supply to neurons.The chapter discusses the energy demands and supply mechanisms of synapses in the brain. It highlights that most brain energy is used on synaptic transmission, emphasizing the importance of understanding how energy is provided and utilized by synapses. The authors review the energy consumption of various subcellular processes, such as ion pumps and neurotransmitter release, and estimate that synaptic transmission consumes a significant portion of the brain's energy. They also explore how pre- and postsynaptic terminals are optimized to maximize information transmission while minimizing energy use, and how ATP provision to synapses is regulated to meet their energetic needs. The chapter further examines how synaptic energy use changes with development, synaptic plasticity, and between wake and sleep states, and discusses the consequences of disrupted energy supply to synapses, including neuropathology. Additionally, it delves into the role of mitochondria in providing energy to synapses and the mechanisms by which astrocytes may contribute to the energy supply to neurons.