Maps of the spatial distribution of different forest types of humid tropical forests are urgently needed for studies of biodiversity, ecology, deforestation and to provide inputs to global circulation models. Recent attempts to map the tropical forests of South America have relied on NOAA-AVHRR data which, with a 1.1 km spatial resolution, is suitable for mapping vegetation at regional and continental scales. Such attempts have, however, generally focused on identifying the forest/non-forest boundary and on determining rates of deforestation and have failed to differentiate different humid tropical forest types. A new sensor -- the ATSR-2 - - launched in 1994 has advantages over the AVHRR. The ATSR-2 has a similar spatial resolution to the AVHRR but its improved radiometric resolution in the visible and near- infrared regions mean that subtle differences in the spectral response of different forest canopies may be detected. In addition, the ATSR-2 has the ability to near-simultaneously view the target area from two different angles providing the scope for reducing atmospheric effects in the data. This capacity may also afford valuable information about the structure of the forest canopy. This paper describes the work currently being undertaken to utilize these characteristics of the ATSR-2 instrument to identify and map humid tropical forests in South America. The project is, in particular, examining how the differences in the (1) phenology, (2) structure, and (3) size, shape and frequency of gaps in the canopy influence the spectral response of different forest types. To date the project has concentrated on determining these parameters for tropical forests in Bolivia and extensive ground observations have been made for different forest plots.