  The MEG experiment, designed to search for the mu+->e+ gamma decay at a
10^-13 sensitivity level, completed data taking in 2013. In order to increase
the sensitivity reach of the experiment by an order of magnitude to the level
of 6 x 10-14 for the branching ratio, a total upgrade, involving substantial
changes to the experiment, has been undertaken, known as MEG II. We present
both the motivation for the upgrade and a detailed overview of the design of
the experiment and of the expected detector performance.
