A study of the vascular flora of the Middlesex Fells Reservation, a suburban forest reserve in greater Boston, was made over nine years from 2003 to 2011. We compiled a checklist, determining the frequency of each taxon and documenting all natural community types occurring in the reservation. We compared our findings to those made in the 1890s, and attempted to explain changes in species composition and frequency. Despite many impacts on the reservation in the last century, a high level of species richness was observed, with 868 vascular taxa extant, including 563 native taxa. There was little net change in the number of native taxa found, but a near tripling of non-native taxa from 110 to 305. Habitat destruction, fragmentation, forest succession, fire suppression, trends favoring mesophytic vegetation, climate change, and invasive species are all believed to be factors in the changes observed in the flora.