This chapter contains instructions for developing your own recipes and/or diagnostics. It also contains a section describing how to use additional datasets with ESMValTool. While it is possible to use just the ESMValCore package and run any recipes/diagnostics you develop with just this package, it is highly recommended that you consider contributing the work you do back to the ESMValTool community. Among the advantages of contributing to the community are improved visibility of your work and support by the community with making and maintaining your diagnostic. See the Community chapter for a guide on how to contribute to the community.