  One important task in the field of flavor physics is to over-constrain the
CKM matrix. To best extract CKM matrix elements, we need inputs from both
experimental data and theoretical calculations. Latest knowledge about the CKM
matrix shows our understanding about sin$2\beta$ is the best thanks to clean
theory and large statistics from experiments. In contrast, our understanding of
both mixing and $|V_{ub}|$ have limitations from theoretical predictions. In
order to extract the value of $|V_{ub}|$ from the branching fraction
measurement, we need information on the hadronic form factor $f_+(q^2)$. Our
best understanding about $f_+(q^2)$ is from lattice QCD calculations. To
validate improved lattice QCD calculations for the form factor in semi-leptonic
B decays, we can use semi-leptonic $D$ decays as a test and calibration. Since
the CKM matrix elements $|V_{cs(d)}|$ can be obtained precisely from unitarity,
we have a reliable method to check the lattice QCD calculations. The
experimental results of $D$ semi-leptonic decays can be summarized in two main
categories: the exclusive decays and the inclusive decays. Latest measurements
from the exclusive semi-leptonic $D$ decays are discussed here.
