Horizontal Branch Morphology and the 2nd Parameter Problem
  We review the most outstanding issues related to the study of the morphology of the Horizontal Branch (HB) in the Color-Magnitude Diagrams of Galactic Globular Clusters and its use as age indicator. It is definitely demonstrated (see also Bolte, this meeting) that age cannot be the only 2nd-P driving the HB morphology. Other candidate 2nd-Ps are briefly examined, with special attention to the possible influence of cluster stellar density. 
F. Fusi Pecci, M. Bellazzini, F.R. Ferraro, R. Buonanno, C. Corsi