Conservation laws, reflecting the symmetry of space and time, play a vital role in understanding the surrounding world. Conservation laws allow us to explain very different phenomena from a unified point of view. The textbook illustrates this principle taking examples from mechanics, optics, nuclear physics, solid-state physics, and medicine. They include, for example, positron annihilation used in experiments aimed at neutrino registration and in the positron emission tomography for patient diagnostics; the functioning of solar cells, infrared detectors, and light emitting diodes (LEDs); slowing down fission neutrons toward achieving a nuclear chain reaction; jet propulsion of a rocket and an octopus; principles of magnetic resonance imaging and principles standing behind fission and fusion nuclear reactions; and more.