  Parafermionic observables were introduced by Smirnov for planar FK
percolation in order to study the critical phase $(p,q)=(p_c(q),q)$. This
article gathers several known properties of these observables. Some of these
properties are used to prove the divergence of the correlation length when
approaching the critical point for FK percolation when $1\le q\le 4$. A crucial
step is to consider FK percolation on the universal cover of the punctured
plane. We also mention several conjectures on FK percolation with arbitrary
cluster-weight $q>0$.
