Abstract Both traditional competition indices and competition kernels are used in many studies to quantify competition between plants for resources. Yet it is not fully clear what the differences between these two concepts really are. For characterising the two approaches we selected two fundamental types of competition indices based on distance weighted size ratios, an additional competition index without distance weighting as a control and developed similar competition kernels. In contrast to the latter approach, competition indices require individual influence zones that for example can be derived from tree crown-radius measurements. We applied these competition measures to two spatio-temporal forest datasets in Europe and one in North America. Stem diameter increment served as observed response variable. The results of both methods indicated similar performance, however, the use of competition kernels produced slightly better results with only one exception out of six computer experiments. Although the performance of both competition measures is not too different, competition kernels are based on more solid mathematical and ecological grounds. Particularly the question of defining the local neighbourhood of a given tree seems to be better handled by competition kernels. Consequently, applications of this method are likely to increase. The trade-off of the use of competition kernels, however, is the need for more sophisticated spatial regression routines that researchers are required to program. Finally, a tabulated summary of differences between competition indices and competition kernels is included.