  Globular clusters are both primary fossils of galactic evolution and
formation and are ideal laboratories for constraining the evolution of low-mass
and metal-poor stars. RR Lyrae and type II Cepheid variables are low-mass,
radially pulsating stars that trace old-age stellar populations. These stellar
standard candles in globular clusters are crucial for measuring their precise
distances and, in turn, absolute ages, and for the calibration of the
extragalactic distance scale. Herein, the evolutionary stages of RR Lyrae and
type II Cepheids are discussed, and their pulsation properties, including the
light curves, color-magnitude and period-amplitude diagrams, and
period-luminosity relations in globular clusters at optical and infrared
wavelengths are presented. The RR Lyrae visual magnitude-metallicity relation
and the multiband period-luminosity-metallicity relations in globular clusters
covering a wide metallicity range are also discussed in detail for their
application to the RR Lyrae-based distance~scale.
