Recent psychophysical and patient work suggests there are independent attentive tracking systems for the left and right visual hemifields. Although neuroimaging has identified cortical regions involved in tracking (e.g., MT and IPS), it remains unclear how these areas contribute to the hemifield effects observed in psychophysical and patient studies. To address this issue, we used fMRI to localize area MT and the area of parietal cortex that is active during tracking (IPS), then used 1Hz repetitive transcranial magnetic stimulation (rTMS) to temporarily suppress activity in area MT or in IPS while observers tracked moving objects. Method: 8 discs moved around (4 in each hemifield). In the unilateral condition, observers tracked 2 items on the left or 2 items on the right. In the bilateral condition, observers tracked 4 items (2 left and 2 right). Unilateral Results: Following rTMS to either MT or IPS, tracking accuracy was relatively worse for targets in the contralateral hemifield than for targets in the ipsilateral hemifield. Bilateral Results: Following rTMS to IPS, the pattern of results was in the same direction as in unilateral condition, but the difference was larger. However, following rTMS to MT, accuracy was greater in the contralateral hemisphere than in the ipsilateral hemisphere. Conclusion: At least two areas within the network underlying attentive tracking (IPS and MT) show robust hemifield effects. Moreover, area MT and IPS are differently involved in tracking; there was a dramatic, qualitative difference between unilateral and bilateral conditions for MT, but only a magnitude difference for IPS.