This Design Guide is intended to provide an introduction to active core sunlighting systems and their implementation for practitioners and stakeholders in the building industry. Active core sunlighting offers an effective approach to energy-efficient lighting by enabling most areas of a building to be illuminated with sunlight during sunshine periods. As a result, efficient and cost-effective active core sunlighting systems have the potential to significantly improve the overall energy efficiency of many buildings. The intent of this document is to lay the groundwork for the eventual adoption of active core sunlighting systems by focusing on the overall performance potential shared by all active core sunlighting systems, rather than focusing on the operational and design details of specific types of products. Examples of some specific systems are offered as a means of generating awareness, but are not intended to be used as detailed design advice or as endorsement of a particular approach.