Most undoped iron-based superconductors show a tetragonal-orthorhombic structural phase transition followed at lower temperature by magnetic ordering, similar to the cuprate superconductors. However, they are poor metals rather than Mott insulators and have five bands at the Fermi surface rather than one. The phase diagram emerging as the iron-arsenide layers are doped is remarkably similar, with the superconducting phase close to or overlapping the magnetic phase. Strong evidence that the T c value varies with the As–Fe–As bond angles has already emerged and shows that the optimal T c value is obtained with undistorted FeAs 4 tetrahedra. The symmetry of the pairing wavefunction is still widely debated, but an extended s -wave scenario is currently favoured.