Nowadays, the vast majority of the tunnel inspection processes are performed manually by qualified operators. The process is subjective and the operators need to face very uncomfortable and even dangerous conditions such as dust environments, absence of light, or toxic substance exposition among others. Robotic technology can overcome many of these disadvantages and provide quality inspections collecting different types of data. This paper presents the key aspects of tunnel inspection and a survey of the developed robotic tunnel inspection systems up to date. Additionally, two projects regarding automation of the processes involved and future trends will be discussed.