This diploma thesis gives an overview of the ventilation systems currently used in tunnels in the construction and operating states, the fluid mechanical principles for designing ventilation systems and the possibilities for measuring the flow velocities in order to ensure the control and functionality of the ventilation systems. In the construction stage, the most important task of ventilation is to supply people working in the tunnel and the combustion engines of the vehicles and machines with fresh air and to convey the exhaust air produced by the tunneling work and the operation of the vehicles out of the tunnel. In the operating state, a constant exchange of air and good visibility conditions must be ensured, especially in road tunnels, due to the combustion-related emissions. For this purpose, different ventilation systems are used in the construction and operating states, depending on the tunnel length, traffic volume and complexity of the tunnel structures, which are described in detail in the first section of this thesis. The design and dimensioning of tunnel ventilation systems is based on basic fluid mechanical equations such as the continuity equation, the momentum equation, the energy equation and the Bernoulli equation. These are used to mathematically describe the air flows in tunnels and the flow losses that occur. Different measuring devices and measuring methods are used to measure the flow velocities in tunnels in the construction and operating states. In the construction state, the air flow velocities are measured to detect leaks, such as damage to air ducts, and to check the functionality of the ventilation system. In the operating state, the ventilation system is automatically controlled by the permanent comparison between ACTUAL velocities and specified TARGET velocities. For the practical part of the thesis, measurements of the flow velocity were carried out at the Zentrum am Berg and at the Gloggnitz construction lot of the Semmering Base Tunnel. Jet fans are installed in all tunnel tubes at the Zentrum am Berg and two axial fans are installed in a building at the portal area of the road tunnel to extract the air via an exhaust duct. The measurement results show that the highest flow velocity in the tunnel can be generated with the two axial fans. The ventilation system at the Gloggnitz construction site is designed as a recirculation ventilation system, in which fresh air is supplied via one tunnel tube and the exhaust air is extracted via the other tunnel tube. The measurement results showed some leakages due to damaged air ducts and open ventilation walls, which were then repaired. In addition to this, the required output of the fans was adjusted in order to permanently guarantee a sufficient supply of fresh air to the working areas.