  We introduce the Yale-Potsdam Stellar Isochrones (YaPSI), a new grid of
stellar evolution tracks and isochrones of solar-scaled composition. In an
effort to improve the Yonsei-Yale database, special emphasis is placed on the
construction of accurate low-mass models (Mstar < 0.6 Msun), and in particular
of their mass-luminosity and mass-radius relations, both crucial in
characterizing exoplanet-host stars and, in turn, their planetary systems. The
YaPSI models cover the mass range 0.15 to 5.0 Msun, densely enough to permit
detailed interpolation in mass, and the metallicity and helium abundance ranges
[Fe/H] = -1.5 to +0.3, and Y = 0.25 to 0.37, specified independently of each
other (i.e., no fixed Delta Y/Delta Z relation is assumed). The evolutionary
tracks are calculated from the pre-main sequence up to the tip of the red giant
branch. The isochrones, with ages between 1 Myr and 20 Gyr, provide UBVRI
colors in the Johnson-Cousins system, and JHK colors in the homogeneized
Bessell & Brett system, derived from two different semi-empirical Teff-color
calibrations from the literature. We also provide utility codes, such as an
isochrone interpolator in age, metallicity, and helium content, and an
interface of the tracks with an open-source Monte Carlo Markov-Chain tool for
the analysis of individual stars. Finally, we present comparisons of the YaPSI
models with the best empirical mass- luminosity and mass-radius relations
available to date, as well as isochrone fitting of well-studied ste
