Concrete floors have traditionally had no problems with annoying vibrations. However with the demand for larger spans the floors need to be made more slender with a higher degree of utilization and lower mass. Therefore new floors may have problems with vibration serviceability.   This thesis studies the vibration characteristics of hollow core slabs. The analysis was made in the finite element program Ansys. The hollow core slab was subjected to a varying force simulating one person walking across the slab. The point where the force was acting on the slab moved through the analysis to simulate the movement of a walking person. Hollow core elements are connected with concrete joints. These joints where modeled in three different ways, two models with hinges and one solid model. The comparison showed that the accelerations caused by the walking person where roughly the same for the models with hinges. In the solid model the amplitudes where slightly smaller.   The calculations show that the largest vibration amplitudes always occur when the forcing frequency is the same as the eigenfrequency of the floor. Also the path of the walking person strongly affects the vibrations in the floor. A path in mid span of a slab will cause larger maximum amplitudes than a path parallel to the hollow core elements. The Maximum Transient Vibration Value (MTVV) will however be larger for paths parallel to one element. Walking along an supported edge creates much lower amplitudes.   This study also shows that increasing the number of hollow core elements alongside of each other decrease the vibration amplitudes linearly with increasing number of elements. The number of contributing elements is also significantly more in the dynamic load case than in the static load case. Therefore load distribution rules from statics directly used in dynamics will be over conservative.