Complex underground structures, such as tunnels, require disciplined and complex work from the very beginning of the design process, through construction, and up to the final stages. The construction of tunnels takes place in geological environments that are not fully known, so changes to the project are always possible. Artificial lighting installed in tunnels must meet many parameters. Along with a precisely designed plan, attention must be paid to the adaptation of the human eye to light. The housing of the lighting fixtures should be resistant to all atmospheric conditions that may occur in the tunnel, thus extending their lifespan. Lighting fixtures in tunnels must ensure safety and visibility for drivers and pedestrians, providing a high level of illumination, while minimizing energy consumption and maintenance needs. An example of designing tunnel lighting in Relux is given.