BRITE-Constellation (BRITE: BRIght Target Explorer) is an international nanosatellite mission to monitor photometrically, in two colours, brightness and temperature variations of stars primarily brighter than mag(V) ~ 4, with precision and time coverage not possible from the ground. The current mission design consists of three pairs of 7 kg nanosats (hence “Constellation ») from Austria, Canada and Poland carrying optical telescopes (3 cm aperture) and CCDs. One instrument in each pair is equipped with a blue filter, the other with a red filter. The prime contractor for the BRITE mission is the University of Toronto Institute for Aerospace Studies – Space Flight Laboratory (SFL). Each BRITE instrument has a wide field of view (~ 24 degrees), so up to about 15 bright stars can be observed simultaneously in 32 x 32 pixel sub-rasters. Photometry with reduced precision but thorough time sampling of additional fainter targets will be possible through on-board data processing. The BRITE sample is dominated by the most intrinsically luminous stars: massive stars seen at all evolutionary stages, and evolved medium-mass stars at the very end of their nuclear burning phases. BRITE-Constellation will primarily measure p- and g-mode pulsations to probe the interiors and ages of stars through asteroseismology, look for rotation of varying spots on the stellar surface, which are e.g. the source of co-rotating interaction regions in the winds of the most luminous stars, and arising probably from newly recognized magnetic subsurface convection and look for planetary transits.