An overview of the possibilities offered when applying Geographical Information Systems (GIS) and Remote Sensing (RS) to environmental monitoring is given, based on examples taken from an ongoing project entitled 'Assessment of mangrove degradation and resilience in the Indian subcontinent: the cases of Godavari estuary and southwest Sri Lanka'. The capabilities of GIS i) to integrate and manage huge amounts of multi-source data, ii) to perform spatial analysis and, iii) to produce synthetic results that can prove useful in decision-making, are highlighted. RS, as a special type of spatial information used in GIS, iv) allows land cover mapping and the inventory of natural resources, v) provides quantitative estimations of biophysical properties of land surface features and, vi) is useful in tracking how landscape changes over time. The examples show that synthetic information and results, such as land cover maps at different dates, aerial estimates of mangrove extension or regression, and surfaces of aquaculture converted from mangroves, paddies or bare lands, can be obtained by analyzing data from different sources in the GIS. Decision-making can make use of such results, but should further benefit from the development of spatial modelling in GIS for the simulation of scenarios. Monitoring capabilities are also improving due to the increase in spatial and spectral resolution of the recently launched remote sensors.