  This short survey, which was written to accompany a minicourse at the BIRS
conference "Topology in dimension 4.5", concerns invariants of knotted
$2$-spheres in $S^4$, also known as $2$-knots. It covers invariants extracted
from the algebraic topology of the knot exterior, including Alexander
invariants, the Farber-Levine pairing and Casson-Gordon invariants, as well as
gauge theoretic and combinatorial invariants. Details are scarce and new
results inexistant.
