The introduction of consumer-level head-mounted devices (HMDs) has led to a major drop in the application costs of virtual reality (VR), making the technology available for a wide range of users. To understand if VR HMDs can be used for planning and training in the context of manual order picking, this thesis provides the results of a large-scale randomized controlled study in which order picking has been compared between a virtual and a real environment. The results imply that VR HMDs can indeed be used by manufacturers and warehouse operators in a rack planning process if the reduction of searching times or the perceived workload is in focus. Additionally, the findings enable the use of VR HMDs for scientific research on human-centred rack design. Finally, the thesis highlights the usability of VR HMDs for training manual order picking activities.