We discuss some recent ACAR results obtained by the Geneva group on YBaCuO, and compare them with the work of West et al and Smedskjaer et al. Next, we present lifetime measurements in both pure YBaCuO, and in Zn doped compound. We also discuss some lifetime calculations which were based on the approach by Jarlborg and Singh. We comment on this approach which is quite intuitive and gives nevertheless results in good agreement with work based on the effect of correlations in the free electron gas. Finally, we comment on the use of positrons for the study of collective effects: ferromagnets, antiferromagnets and superconductors. We also mention the need for better equipment.