Knowledgeable exposure is the core skill of quality photography. Achieving the right combination of light and shadow that matches, or even amplifies, our artistic vision is the essence of success. This chapter focuses on what we need to know about photographic exposure to truly master landscape astrophotography. The primary photographic challenge, especially in landscape astrophotography, is to collect the right amount of light to produce a properly exposed image, regardless of the amount of light originating from the scene. The ISO setting refers to the overall sensitivity of the camera sensor to light, and dictates how much light is needed to obtain a properly exposed image, or one that has an overall gray level of 18 percent. With reference to the firefly analogy, the camera exposure value is a measure of how many fireflies need to land on the sensor in order to make the image.