We are developing the BARC (Bead ARray Counter) sensor chip for the detection of biomolecules labeled with magnetic microbeads.@footnote 1@ Presently, 2.8 µm-diameter commercial magnetic beads are detected by an array of 64 GMR sensors on the chip, with each sensor spanning a 200 µm diameter spot. Arrays of single-stranded DNA or antibody probes are immobilized onto the sensor spots, and biomolecular targets (e.g. DNA or proteins) that are captured by the probes are then labeled with magnetic microbeads. Although at the limits of detection each bead labels a single captured molecule, non-specifically bound beads and sensor noise currently set the limit of detection to about 10 beads (potentially 10 molecules) per sensor. Although the sensor signal to noise can be improved, ultimately the sensitivity will be limited by delivery of molecules to the sensor surface as governed by diffusion, sensor geometry, and fluidics. The current BARC sensor array has an advantage because of its relatively large sensors, and can presently detect DNA concentrations as low as 1 fM (10@super 5@ molecules/cm@super 3@). Further improvement in the sensitivity will require coupling the design of the fluidics with the sensor array. I will discuss the BARC sensor response, along with finite element calculations of the delivery of molecules to the surface under various conditions. I will also discuss how these issues affect various alternative magnetic labeling and detection approaches, such as those based on spin valves and SQUIDs. This work done in collaboration with M. M. Miller, P. E. Sheehan, C. R. Tamanaha, M. Tondra, and L. J. Whitman. @FootnoteText@ @footnote 1@J. C. Rife et al., Sensors and Actuators A 107, 209-218 (2003).