  The newly discovered iron-based high temperature superconductors have
demonstrated rich physical properties. Here we give a brief review on the
recent studies of the upper critical field and its anisotropy in a few typical
series of the iron-based superconductors (FeSCs). In spite of their characters
of a layered crystal structure, all the FeSCs possess an extremely large upper
critical field and a weak anisotropy of superconductivity, being unique among
the layered superconductors. These particular properties indicate potential
applications of the FeSCs in the future. Based on the experimental facts of the
FeSCs, we will discuss the possible mechanisms of pair breaking in high
magnetic fields and its restrictions on the theoretical analysis of the
superconducting pairing mechanisms.
