  We review a series of unitarization techniques that have been used during the
last decades, many of them in connection with the advent and development of
current algebra and later of Chiral Perturbation Theory. Several methods are
discussed like the generalized effective-range expansion, K-matrix approach,
Inverse Amplitude Method, Pad\'e approximants and the N/D method. More details
are given for the latter though. We also consider how to implement them in
order to correct by final-state interactions. In connection with this some
other methods are also introduced like the expansion of the inverse of the form
factor, the Omn\'es solution, generalization to coupled channels and the
Khuri-Treiman formalism, among others.
