This presentation shows how to better integrate simulation codes and Geographical Information Systems, and takes the example of Hydrological modelling integration into QGIS. Scientific modelling and simulations are present in a large number of areas. A significant proportion of simulation codes are applied spatially, at different levels, from a neighborhood scale up to worldwide areas. These simulation codes take spatial information as input data, and output results which are related to space too. But most of the time, they do not directly handle GIS data. Data types and data formats are different, and there is therefore a lot of effort to put into pre-processing and post-processing of the data to get it from GIS to the simulation codes and back. For example, determining the diffusion of a pollutant leak into underground water necessitates to get a DEM, location of the leak, geological data and more from the GIS, and transform it to simulation code input format. Then launch a simulation (on finite volumes e.g.), and convert the output into GIS files so that to be able to visualize spatial repartition of the pollutant according to time. The topic of this presentation is therefore to show how to better interact between simulation and GIS. We present the prevalent types of data for simulation, how they differ from GIS, and how we usually transfer from one type to another. Then we show how we worked towards better integration. Polygonal meshes are the most common way of representing 2D geometries for simulation purposes. Integrating simulation to a GIS requires storing georeferenced meshes in a databases (or using standard GIS file formats), and being able to use simulation values interpolated over the elements as a map layer. We show how to modify simulation codes to read directly a mesh from a GIS and write the results into a GIS. We implemented a new type of layer for QGIS, a mesh layer, which enables to display simulation results with high performances. This takes into account the temporal dimension. We also demonstrate how to integrate a simulation code into QGIS Processing so that it can be managed directly from within the desktop application. We illustrate these concepts with a demonstration of a full integration of a Hydrological simulation tool inside QGIS, with simulation management, custom user interface and strong integration of data between the simulation code and GIS data. In this sense the FREEWAT project started mid-2015, which aims at integrating multiple Hydrological codes into QGIS is also a good example of simulation and GIS integration. We end up with the perspectives for more global integration of simulation tools and GIS, and the work still to be done to bridge the gap between those two worlds.