  This is a colloquium-style introduction to two electronic processes in a
carbon monolayer (graphene), each having an analogue in relativistic quantum
mechanics. Both processes couple electron-like and hole-like states, through
the action of either a superconducting pair potential or an electrostatic
potential. The first process, Andreev reflection, is the electron-to-hole
conversion at the interface with a superconductor. The second process, Klein
tunneling, is the tunneling through a p-n junction. Existing and proposed
experiments on Josephson junctions and bipolar junctions in graphene are
discussed from a unified perspective.
  CONTENTS:
  I. INTRODUCTION
  II. BASIC PHYSICS OF GRAPHENE (Dirac equation; Time reversal symmetry;
Boundary conditions; Pseudo-diffusive dynamics)
  III. ANDREEV REFLECTION (Electron-hole conversion; Retro-reflection vs.
specular reflection; Dirac-Bogoliubov-de Gennes equation; Josephson junctions;
Further reading)
  IV. KLEIN TUNNELING (Absence of backscattering; Bipolar junctions; Magnetic
field effects; Further reading)
  V. ANALOGIES (Mapping between NS and p-n junction; Retro-reflection vs.
negative refraction; Valley-isospin dependent quantum Hall effect;
Pseudo-superconductivity)
