The NASA Curiosity rover Mastcam system is a pair of fixed-focal length, multispectral, color CCD imagers mounted ~2 m above the surface on the rover's remote sensing mast, along with associated electronics and an onboard calibration target. The left Mastcam (M-34) has a 34-mm focal length, an IFOV of 0.22 mrad, and a FOV of 20°⨉15° over the full 1648⨉1200 pixel span of its Kodak KAI-2020 CCD. ... [Show full abstract] The right Mastcam (M-100) has a 100-mm focal length, an IFOV of 0.074 mrad, and a FOV of 6.8°⨉5.1° using the same detector. The cameras are separated by 24.2 cm on the mast, allowing stereo images to be obtained at the resolution of the M-34 camera. Each camera has an 8-position filter wheel, enabling it to take Bayer pattern RGB "true color" images, multispectral images in nine additional bands spanning ~400-1100 nm, and images of the Sun in two colors through neutral-density-coated filters. An associated Digital Electronics Assembly provides command and data interfaces to the rover, 8 GBytes of image storage per camera, 11-bit to 8-bit companding, JPEG compression, and acquisition of high-definition video. Here we describe the pre-flight and in-flight calibration of Mastcam images, the ways that they are being archived in the NASA Planetary Data System, and the ways that calibration refinements are being developed as the investigation progresses on Mars. We also provide some examples of data sets and analyses that help to validate the accuracy and precision of the calibration.