  I discuss how to obtain angular decay distributions for sequential cascade
decays using helicity methods. The angular decay distributions follow from a
reasonably simple master formula involving bilinear forms of helicity
amplitudes and Wigner's $d$ functions. I discuss in some detail the issue of
gauge invariance for off-shell gauge bosons. As a technical exercise I
calculate the linear relation between the helicity amplitudes and the invariant
amplitudes of semileptonic and rare baryon decays. I discuss two explicit
examples of angular decay distributions for (i) the decay $t\to b+W^+(\to
\ell^+\nu_\ell)$ (which leads to the notion of the helicity fractions of the
$W^+$), and (ii) the sequential decay $\Lambda_b\to\Lambda(\to
p\pi^-)+J/\psi(\to \ell^+\ell^-)$.
