  An analysis of the observed characteristics of the Galactic Cepheid variables
is carried out in the framework of their period-luminosity relation being used
as a standard candle for distance measurement. The variation of the observed
number density of Galactic Cepheids as function of their period and amplitude
along with stellar pulsation characteristics is used to divide the population
into two groups: one with low periods, probably multi-mode or higher mode
oscillators, and another of high period variables which should be dominantly
fundamental mode radial pulsators. Methods to obtain extinction-corrected
colors from multi-wavelength observations of the second group of variables are
described and templates of the (V-I) light curves are obtained from the V light
curves. Colors computed from the model atmospheres are compared with the
extinction-corrected colors to determine the Cepheid instability strip in the
mean surface gravity--effective temperature diagram, and relations are derived
between mean colors (B-V) vs period of pulsation, (V-I) vs period, and (V-I) at
the brightest phase vs amplitude of pulsation. The strength of the
kappa-mechanism in the envelope models is used to estimate the metal dependency
of the instability strip from which an idea of the sensitivity of the
period-luminosity relation to the helium and metal abundance is given. Some
estimate of the mass of Cepheids along the instability strip is provided.
