  In this paper, we will give an overview of known and new techniques on how
one can obtain explicit equations for candidates of good towers of function
fields. The techniques are founded in modular theory (both the classical
modular theory and the Drinfeld modular theory). In the classical modular
setup, optimal towers can be obtained, while in the Drinfeld modular setup,
good towers over any non-prime field may be found. We illustrate the theory
with several examples, thus explaining some known towers as well as giving new
examples of good explicitly defined towers of function fields.
