Pulsating variable stars such as Cepheid and RR Lyrae are primary standard candles for the extra-galactic distance measurements. One of the most fascinating aims of contemporary variable star research is to strengthen the first-rung of the cosmic distance ladder which allows a precise measurement of the Hubble constant in the late evolutionary Universe. At present, a ~2% local measurement of the Hubble constant based on Cepheids and type Ia Supernovae distance ladder is significantly (~9% or ~4σ) larger than its precise Planck mission value from the early Universe. This tension in the expansion rate between the two extreme ends of the Universe hints at possible new physics beyond the standard cosmological model. However, a less-exciting prospect is that the tension between the two measurements is possibly due to their known or unknown systematic uncertainties. I will discuss ongoing global efforts to reduce Cepheid-based systematics in the distance ladder and provide independent distance measurements based on pulsating stars.
I will present my work on improving the primary calibration of the distance ladder based on Cepheid, Miras, and RR Lyrae variables. I will discuss near-infrared pulsation properties of RR Lyrae variables in detail for the first time in the Messier 3 and Messier 53 globular clusters as part of our ongoing program to constrain the metallicity effects on their Period-Luminosity relations. We will also present results on maximum-light properties of Mira variables in the Magellanic Clouds and their Period-Luminosity relations at maximum light, which exhibit significantly smaller scatter than the classical mean-light relations. The maximum-light properties of Miras will be very useful for stellar atmosphere modeling and distance scale studies provided their stability and the universality can be established in other stellar environments in the era of extremely large telescopes. Finally, I will briefly discuss the application of population II Cepheids as stellar population tracers of structure and kinematics of old stellar populations in the Galactic bulge.