This study evaluates the potential of measuring/mapping forest decline in spruce-fir forests using airborne NS-001 Thematic Mapper Simulator (TMS) data. Using field instruments, it was found that ratios of 1.65/1.23 and 1 65/083 μm reflectance discriminated between spruce samples of low and high damage sites. Using TMS data, 1.65/1.23 and 1 65/0.83 μm band ratios were found to be strongly correlated with ground-based measurements of forest damage. Ratio colour density slice images using 1.65/1.23 or 1.65/0.83μm band ratios, and images using 0.56μ and 1.65μm bands with either of these band ratios in a false colour composite, provide accurate means of detecting, quantifying and mapping levels of forest decline. Using these techniques, new areas of forest decline have been detected, and monitoring of forest decline will be possible.