Recent years have revealed a variety of mathematical and physical structures underlying scattering amplitudes, with redefinitions of scattering amplitudes where the usual principles of locality and unitarity are derivative from geometry. The amplituhedron is one of the examples, a purely geometric object which gives scattering amplitudes of planar N=4 SYM. All tree-level amplitudes and all-loop integrands correspond to the differential forms with logarithmic singularities on the boundaries of the amplituhedron. In this lecture, I will give a review of scattering amplitudes in planar N-4 SYM first. After this, I will explain the definition of the amplituhedron and see how to obtain scattering amplitudes from this geometric object.