This  volume  is  based  on  the  PhD  thesis  of  the  author.
Through the examples of the self-avoiding walk, the random-cluster model,
the  Ising  model  and  others,  the  book  explores  in  details  two  important
techniques:
1.Discrete  holomorphicity  and  parafermionic  observables, which have
been used in the past few years to study planar models of statistical
physics (in particular their conformal invariance), such as
random-cluster models and loop O(n)-models.
2.  The Russo-Seymour-Welsh  theory for percolation-type models with
dependence.    This  technique  was  initially  available  for  Bernoulli
percolation  only.   Recently,  it  has  been  extended  to  models  with
dependence, thus opening the way to a deeper study of their critical
regime.