In this NASA image (high-res version) captured from the International Space Station (ISS), the top of the blue band marks what is often called the edge of Earth's atmosphere, about 100 km up. Note the moon, faintly visible in the left-center. To learn more, visit The Top of the Atmosphere.
The altitude of the ISS ranges from about 350 km to 450 km. Even at this height, atmospheric drag slowly degrades the orbit, so periodic boosts are required (see NASA's S'Mores), especially when intense solar activity puffs out parts of the atmosphere.