Literature data on (self-)diffusion in transition metal borides are extremely sparse due to the low atomic mobility of the constituents and due to the fact that for B there exist no suitable radioactive tracers and only two stable isotopes with a high natural abundance of 19 % (10B) and 81 % (11B), respectively. The present paper reviews our experiments on the tracer diffusion of transition metals and boron in TiB2, WB2+x, and (TixWyCrz)B2 which were carried out using stable isotopes and secondary ion mass spectrometry (SIMS). For tracer deposition, ion implantation and magnetron sputtering were used. In order to measure boron diffusion, a specially designed experiment was build up where a TiB2 layer was sputtered on an isotope-enriched Ti11B2 bulk ceramic sample. In addition, first results on chemical interdiffusion in the system (TixWyCrz)B2 will be presented. Here, a method based on magnetron sputtered layers and secondary neutral mass spectrometry (SNMS) was used which allows to determine much lower diffusivities (down to 10-19 m2/s) than the conventional EDX line-scan method on cross-sectional samples.