This manual addresses the design of major pipeline or duct segments to be installed by horizontal directional drilling (HDD). The manual, which covers topics specifically related to HDD installation, was prepared by a committee of senior engineers who are leaders in the development of HDD techniques and practices. HDD is a trenchless excavation method that is accomplished in three phases and uses a speciliazed drilling rig with ancillary tools and equipment. This manual is meant to be a guide for design engineers with previous experience and knowledge of the HDD installation process and pipeline design methods.