The outline of this section is as follows.In Section III.C.1 the experimental results on coherent electron focusing59,80 are presented. A theoretical description80,347 is given in Section III.C.2, in terms of mode interference in the waveguide formed by the magnetic field at the 2DEG boundary. Apart from the intrinsic interest of electron focusing in a 2DEG, the experiment can also be seen as a method to study electron scattering, as in metals. Two such applications108,349 are discussed in Section III.C.3. We restrict ourselves in this section to focusing by a magnetic field. Electrostatic focusing350 is discussed in Section III.D.2.