The MEG experiment, designed to search for the μ ^+ →e^+ γ decay, completed data-taking in 2013 reaching a sensitivity level of 5.3× 10^-13 for the branching ratio. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6× 10^-14 , 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.
