Atomic Force Microscope images and other experiments show us that very small stable bubbles, known as nanobubbles, can be present on surfaces despite well founded theoretical considerations that predict otherwise. Nanobubbles are thought to play a role in the rupture of thin films during froth flotation, hydrodynamic slip over surfaces, interaction forces between hydrophobic surfaces and influence the electroplating and electrolysis processes. Here we describe what is known of nanobubbles and discuss the challenges in understanding nanobubble morphology and stability.