Current problems on horizontal branch stars

In the light of our present knowledge on stellar evolution, it is shown that red ZAHB will not develop for very low metal Globular Clusters ifY is appreciably greater than 0.1.Recent determinations of the mass-sizes of the He-cores at the time of the He-flash support independently conclusion that, if no increase in surface He-abundance occurs between the turn-off evolutionary phase and the H.B. stages,Y-values lower than 0.2 imply ages of the clusters larger than 20 b.y. By comparing evolutionary paths with the observed populations of the Horizontal Branches we find evidence for mass loss in blue H.B., and evidence for mass dispersion in red H.B. not depopulated in RR Lyrae. A few clusters (M3, M15) have their H.B. dominated by mass dispersion. Finally the expected correlations between mass loss and spatial distribution of H.B. stars are briefly discussed in order to point out some observational possibilities.