Potential positive feedbacks of the biosphere on glacial-interglacial climate change have been extensively investigated in recent years. In this paper, we summarize these feedbacks and the evidence that they may play a quantitatively significant role. We then attempt to assess the role of biosphere feedbacks in glacial/interglacial climate change by evaluating five lines of empirical evidence: (1) synchroneity of warming during the last glacial termination (expected if the biosphere is important, because of short response times); (2) changes in the δ18O of O2, which may reflect the relative fertility of the land and ocean biospheres, (3) changes in the triple isotope composition of O2, which constrain global rates of photosynthesis in the past; (4) the relationship between atmospheric CO2 and dust accumulation at Vostok, and (5) indications for the occurrence or absence of Pleistocene-style glacial cycles before the evolution of the land biosphere. The evidence is compatible with a significant role for the biosphere in driving glacial-interglacial change, but unambiguous empirical support is not yet in hand.