The formation of the Si II resonance multiplets near 1814, 1530, 1306 and 1262 $\AA$ in the Sun is investigated by means of non-LTE radiative transfer computations. Use has been made of recent calculations for electron-impact excitation rates and absorption oscillator strenghts. The lines of the multiplet near 1814 $\AA$ are found to be effectively thin, but ther depth of formatiom embraces regions of widely differing gas pressures, essentially invalidating the {\it coronal} approximation. Furthermore, the Si II (1807 $\AA$) line is formed at deeper and cooler region than the Si II (1808$\AA$) and Si II (1816$\AA$) lines. The lines of the other multiplets are found effectively thick, so that an accurate evaluation of their emerging flux requires a non-LTE radiative transfer treatment, ideally under partial re-distribution approximation. The important diagnostic properties of these lines for the upper chromosphere and lower transition region are discussed. The intensities of Si II (1526.7$\AA$) and Si II (1533.4$\AA$) are compared with observational data from the High-Resolution Telescope and Spectrograph (HRTS), while, for the other lines, line ratios are compared with observations from HRTS and from the Naval Research Laboratory slit spectrograph on {\it skylab}.
Key words: Sun: Chromosphere - Sun: transition region - Sun: UV radiation - Line: formation