  We investigate statistical equilibrium of neutral and singly-ionized
strontium in late-type stellar atmospheres. Particular attention is given to
the completeness of the model atom, which includes new energy levels,
transition probabilities, photoionization and electron-impact excitation
cross-sections computed with the R-matrix method. The NLTE model is applied to
the analysis of Sr I and Sr II lines in the spectra of the Sun, Procyon,
Arcturus, and HD 122563, showing a significant improvement in the ionization
balance compared to LTE line formation calculations, which predict abundance
discrepancies of up to 0.5 dex. The solar Sr abundance is log A = 2.93 \pm 0.04
dex, in agreement with the meteorites. A grid of NLTE abundance corrections for
Sr I and Sr II lines covering a large range of stellar parameters is presented.
