Cosmological numerical simulations suggest that dSphs and dEs formed hierarchically via assembling much smaller stellar/gas clumps. Star formation had occurred intensively during the building up phase that is shorter than ∼1 Gyr. Dark matter assembled hierarchically, while gas collapsed dissipatively; as a result the metallicity gradient was built up and stars at the galaxy centre tend to have larger rotational velocity and smaller velocity dispersion. There are increasing evidences, however, that suggest rather long-lasted star formation in dwarf galaxies with much smaller SFRs, with which cosmological simulations need to be reconciled.