Adaptive grid refinement in Fortran (AGRIF) is a Fortran90 package for the integration of adaptive mesh refinement (AMR) features within existing finite difference codes. The package first provides model-independent Fortran90 procedures containing the different operations in an AMR process: time integration of grid hierarchy, clustering, interpolations, updates, etc. The package then creates the Fortran90 model-dependent part of the code based on an entry file written by the user. The basic idea of AGRIF is to make use of Fortran90 pointers to successively address the variables of the different grids of an AMR process. As pointers can be used exactly like other (static) variables in Fortran, most of the original code will remain unchanged.