The interactive visualization of very large macromolecular complexes on the web is becoming a challenging problem as experimental techniques advance at an unprecedented rate and deliver structures of increasing size. We have tackled this problem by introducing the binary and compressed MacroMolecular Transmission Format (MMTF) to reduce network transfer and parsing time, and by developing NGL,a highly memoryefficient and scalable WebGL-based viewer. NGL renders molecular complexes with millions of atoms interactively on desktop computers and smartphones alike, making it a tool of choice for web-based molecular visualization in research and education.