  The most recent determinations of $V_{ud}$ from superallowed beta decays lead
to a discrepancy when compared to the value implied by mesonic CKM measurements
combined with CKM unitarity. On top of this, improved precision in lattice QCD
calculations have revealed another discrepancy between the $V_{us}$
determinations from kaon and pion semi-leptonic decays. The combination of
these can be referred to as the Cabibbo angle anomaly, which we find has a
significance of around $3\,\sigma$. After summarising the current state of
these issues, I will talk about new physics models that modify semi-leptonic
decays as potential explanations, and why vector-like quarks in particular
appear the most promising candidates. I will then discuss the results of a
global fit to various vector-like quark models, and how other constraints are
important in determining the most likely explanation. Finally I will touch on
future experiments that could shed further light on the situation.
