Dendrites are long branching structures of neurons that can extend over long distances inside the brain. While the conventional view was that dendrites act as simple antennas of neurons, an emerging view claims that they are capable of more active functions, such as boosting the computational power of the neuron. This thesis examines the role of dendrites inside the mouse brain, using red light to silence dendritic activity via the genetic expression of light-sensitive ion pumps and observe its effect. It demonstrates that dendrites indeed have a significant role in shaping the final output of single neurons and are important functional components of the brain.