I will discuss nonlinear and convective theoretical models for
radial pulsating stars that populate the 'Classical Instability
Strip' (IS). In particular I will concentrate on the pulsation
properties of an important class of radially pulsating stars:
Classical Cepheids. A brief overview on the origin of the
relationships between the stellar physical parameters, namely
mass, luminosity and effective temperature, and the pulsation
period of Cepheids will be discussed. These relations
(Period-Luminosity-Color, Wesenheit and Period Luminosity) allow
us to use Cepheid stars as primary distance indicators and to
calibrate fundamental secondary distance indicators such as the
peak luminosity of  the SN Ia. Particular care will be devoted to
the still debated problem of the universality of the PL relation,
as well as to the evidences of its nonlinearity for periods longer
than 10 days.
I will show also the final result, obtained with our theoretical
models, on the distance modulus of a very striking object like the
Blue Compact Dwarf galaxy: IZw18.