Lightning discharges have broad impacts on the Earth's mesosphere, ionosphere, magnetosphere, and radio environment. The study of radio waves of ELF/VLF frequencies (300 Hz - 30 kHz) has long proven fruitful in remotely sensing both the lightning discharges, as well as its various geophysical consequences. ELF/VLF waves are highly reflected by the D region of the ionosphere (60-90 km), thereby making them uniquely suited for continuous monitoring of a region too low for satellites, yet too high for hot air balloons. The D region is disturbed not only by lightning but by solar flares, electron precipitation, earthquakes, gamma-ray bursts, and others. ELF/VLF waves are also guided efficiently to global distances, making them an important component of naval communications and global navigation. We will review the current state of ELF/VLF research, including detection of lightning-generated radio signals, ELF/VLF signals generated in the Earth's magnetosphere, and VLF remote sensing of the D-region of the ionosphere. We will also describe the role of ELF/VLF receiver network development and scientific capacity building in the NASA-sponsored International Heliophysical Year (IHY) and International Space Weather Initiative (ISWI).