WHO has made a significant contribution by producing successive revisions of the semen analysis manual. The 5th edition to be published soon has a number of improvements with clearer and simpler instructions for standard analysis particularly for sperm concentration and motility. Morphology continues to be done by the strict method. There are tables for comparing duplicates. New reference values (weighted 5th percentiles) from about 1600 fertile men (recent fathers) from different countries are included. The section on sperm preparation has been expanded to cover common methods of isolation of sperm for assisted reproductive technology. A chapter on sperm cryopreservation has been added. The quality control chapter has been rewritten. There is less material in appendices but there is a new appendix on the basics of microscopy. The aim of the manual is to standardise semen analysis but experience with external quality assurance indicates this is difficult to achieve. This is particularly the case with sperm morphology. Clinicians will continue need to know the reference ranges for their own laboratories. We have experience with routine use of computer assisted semen analysis (CASA) for sperm concentration, percent progressively motile sperm and sperm kinematics: straight line velocity (VSL), using the Hamilton Thorn CASA with IDENT fluorescent staining of the sperm. This requires dilution of samples with high sperm Abstract of the 8th Royan International Twin Congress, Tehran, Iran, 5-7 September 2007 Yakhteh Medical Journal, Vol 9, Sup 1, Summer 2007 9 concentration with seminal plasma and selection of fields across the counting chamber to achieve accuracy. For sperm concentrations above 2x106/mL the quality control is very good. We have a sperm morphometry system based on automated focusing and slide movement that provides 32 morphometric measurements of the sperm head and upper neck regions including features related to density of staining that allow orientation of the sperm head and assessment of the acrosomal region. An index (%Z) derived from features selected by the sperm-ZP binding process was related to natural conception rates in about 1200 subfertile couples. VSL and female age were also significant in regression analysis models. We believe these and other advances in CASA will greatly improve semen analysis.