This book summarizes the recent research and development in the field of glass micro- and nanospheres. With special focus on the physics of spherical whispering-gallery mode resonators, it presents selected examples of application of glass microspheres in biosensing, laser devices, and microwave engineering. Hollow microspheres also offer a perspective for hydrogen transport and storage. On the other hand, glass nanospheres are fundamental for a class of photonic crystals (e.g., direct and inverse opals), as well as for industrial composite materials. Both micro- and nanospheres find important applications in biomedicine. The book highlights examples of preparation techniques and applications, addresses recent challenges, and examines potential solutions. It addresses physicists, chemists, materials scientists, and engineers, working with glass materials on microcavities, on nanotechnologies, and on their applications.