  We explore the origin of the observed Lya and other UV lines from galaxies at
z<3.7 by detailed modelling of the spectra. The objects are chosen among those
showing a) UV-optical-near-IR lines, b) only UV lines and c) those showing Lya
in the UV and a few optical lines. We also present UV line predictions for a
sample of galaxies in the 0.0686<z<0.8829 range. The sample of galaxies
including Lya observations in their spectra does not show peculiar physical
conditions of the emitting gas, nor abnormal element abundances. However, the
high velocity (Vs>1000 km/s) component of the emitting gas is accompanied by
relatively low preshock densities (n0 ~100-400 cm^-3) leading in some cases to
broad forbidden lines. Some spectra are best reproduced by shock dominated
models in which the photoionizing source is hidden or absent. Within more than
50 galaxies modelled in this work, only a few spectra from galaxies at z~2.5
correspond to a starburst temperature Ts>10^5 K, similar to that found in
galaxies showing some activity.
