The spiral structure of the Milky Way
  The morphology and kinematics of the spiral structure of the Milky Way is a
long-standing problem in astrophysics. In this review we firstly summarize
various methods with different tracers used to solve this puzzle. The
astrometry of Galactic sources is gradually alleviating this difficult
situation caused mainly by large distance uncertainties, as we can currently
obtain accurate parallaxes (a few $\mu$as) and proper motions ($\approx$ 1
km~s$^{-1}$) by using Very Long Baseline Interferometry (VLBI). On the other
hand, Gaia mission is providing the largest, uniform sample of parallaxes for
O-type stars in the entire Milky Way. Based upon the VLBI maser and Gaia O-star
parallax measurements, nearby spiral structures: the Perseus, Local,
Sagittarius and Scutum arms are determined in unprecedented detail. Meanwhile,
we estimate fundamental Galactic parameters, the distance to the Galactic
center, $R_0$, to be 8.35 $\pm$ 0.18 kpc, a circular rotation speed at the Sun,
$\Theta_0$, to be 240 $\pm$ 10 km~s$^{-1}$. We found kinematic differences
between O stars and interstellar masers: the O stars, on average, rotate
faster, $>$~8~ km s$^{-1}$ than maser-traced high-mass star forming regions.
