The feasibility of multitemporal special sensor microwave imager (SSM/I) data for monitoring boreal forests was evaluated. The parameters of interest were forest coverage fraction and forest stem volume (biomass). The employed test sites covered almost the whole of Finland. Two measurement periods were used: July through September in 1993 and January through February in 1994. The apparent emissivities of various land cover types were determined with the mixed pixel approach under summer and winter conditions. The aim was to define the dominating factors on emissivity under winter and summer conditions. The mixed pixel approach was tested for the estimation of forest coverage fraction. The results with multitemporal SSM/I data show that the pixel-wise fractions of water, nonforested, and forested area can be estimated with rms errors of around 10 percent units. The correlation between the estimates and the ground truth was over 0.85. A new inversion method for stem volume estimation was presented. At SSM/I frequencies the forest canopy and the snow-covered ground dominate the emissivity behavior under winter conditions in the boreal forest zone. The method is based on the fact that the emissivity of forest canopy is close to 1, while that of dry snow cover is relatively low. The results with the method showed promising accuracies when wintertime SSM/I data were employed. The rms error was from 13 to 19 m3/ha per pixel (25 km by 25 km), which was 15–16% of the mean stem volume. In the test area the stem volume ranged from 40 to 160 m3/ha per pixel.