What is the cosmological role of dSphs? With regard to the oldest measurable ages and the earliest epoch of star formation, we find consistency with expectations from cosmological modes. There appears to be a common epoch of early star formation in the Milky Way and in its dwarf companions. In contrast to model predictions, the expected cessation of star formation after reionization is, however, not observed in dSphs. The observed population structure in the (very different) halos of M31 and of the Milky Way, makes it seem unlikely that (present-day) dSphs played a major role in the build-up of the halo of the two spirals. The large variations in the star formation histories of dSphs and the presence of younger populations than in the Galactic halo can be reconciled with the building block scenario if most of the accretion events occurred very early on.
The global metallicities of the Milky Way dSphs are well-matched to those observed in the Galactic halo, but this is not the case for the M31 dSph companions. With regard to detailed chemical element abundance ratios, it is emerging that dSphs cannot have been dominant contributors to halo build-up unless - again - the merger events would have taken place at very early times. The differences in the metallicity-luminosity relation of different types of dwarfs seem to exclude that dSphs are simply stripped dIrrs.
Both disruption and accretion of dwarf companions are still occurring today, demonstrating that dSphs must have played some role in the growth of larger galaxies. Unfortunately, the number and importance of accretion events remains unclear for either of the two large spirals in the Local Group.
In spite of admirable progress, dwarfs remain an evolutionary puzzle. They are excellent and important test cases of cosmological predictions. Regardless of their cosmological importance, however, dwarf galaxies are also interesting in their own right!
Acknowledgments Many thanks to Helmut Jerjen and Bruno Binggeli for a wonderful conference and for their patience while this contribution was finished. I am also indebted to Jay Gallagher for a critical reading of this text.