The purpose of this report is to provide information on the lighting requirements for drivers approaching the entrance of a tunnel in daytime. The report is restricted to the first stretch of tunnels, called the threshold zone. The report begins with a review of the fundamental experiments that were the basis of the 1973 CIE recommendations for tunnel lighting, as far as the threshold zone is concerned (see CIE 23-1973 International Recommendations for Motorway Lighting). New investigations are described with which the original fundamental experiments may be applied more realistically. Results of driving experiments in a real tunnel under various lighting conditions are summarized. To complete the description of methods based on visibility, two recently published methods that dispense with the original fundamental experiments are discussed. Finally, mention is made of an approach not directly based on visibility but on traffic flow. The report does not give a straightforward answer to all problems related to the lighting of threshold zones of tunnels. It summarizes the state of the art and, as such, will form the background for future tunnel lighting recommendations.