The book "Hydromagnetic Waves in the Magnetosphere and the Ionosphere" offers a comprehensive examination of Ultra-Low-Frequency (ULF) electromagnetic waves observed both on Earth and in space. These waves, also known as geomagnetic variations or pulsations, result from Alfvén's discovery, which describes the influence of a strong magnetic field on conducting fluids. The development of the concept of magnetospheric magnetohydrodynamic (MHD) waves is explored in detail. The book highlights the application of these waves for real-time monitoring of the magnetosphere as well as for deep electromagnetic penetration of the Earth. It demonstrates how ULF waves can gather information about the magnetosphere, the ionosphere, and the Earth's surface. These insights are of great significance to geophysicists, as they offer new opportunities for exploring the solid Earth and its gas-plasma envelopes.