  In this paper, we show that the total area of two distinct surfaces with
Gaussian curvature equal to 1, which are also conformal to the Euclidean unit
disk with the same conformal factor on the boundary, must be at least 4{\pi}.
In other words, the areas of these surfaces must cover the whole unit sphere
after a proper rearrangement. We refer to this lower bound of total area as the
Sphere Covering Inequality. The inequality and its generalizations are applied
to a number of open problems related to Moser- Trudinger type inequalities,
mean field equations and Onsager vortices, etc, and yield optimal results.
