These notes are the pdf slides that were used in the workshop entitled: "Numerical Modelling of Chemical Diffusion in Petrology and Geochemistry" that was held in Mainz in July, 2023. Apart from the lecture material, I have provided code examples that the interested readers can copy and use for their own research. These notes do not cover the diffusion theory extensively, and the interested reader should refer to available textbooks (see the reference list at the end of file). The codes and the rest of the material are provided with no warranty. They are mostly done for educational purposes and assume no programming experience on behalf of the user. Therefore, to more experienced programmers, these codes may look redundant. Since this is the first version of these notes it is possible that these notes contain errors. Instead of waiting until I complete the 'perfect' notes I decided to proceed with the material that I already have. Any constructive feedback and comments are welcome and should be addressed to evmoulas@uni-mainz.de. I will try to update the notes every year depending on the requests that I get from the workshop participants or other interested readers. In case you find these notes useful, please cite them appropriately (see rules at https://zenodo.org/). Writing these notes and the organization of the workshop would not have been possible without the support from the German Mineralogical Society (DMG) and the Mainz Institute of Multiscale Modeling (M3ODEL). The institute of Geosciences in Mainz is acknowledged for hosting the workshop and Claudia Scheer is thanked for providing valuable assistance. I would also like to thank Lucie Tajčmanová, Roman Botcharnikov, Sumit Chakraborty and Boris Kaus for encouraging me to organize this workshop. Finally, I would like to thank Simon Boisserée and Annalena Stroh for going through this material and for providing feedback in advance. Evangelos Moulas