We have developed a broadly applicable biosensing platform based upon chip-integrated silicon photonic microcavities. Large arrays of microring resonators are robustly fabricated using standard semiconductor processing methods on silicon-on-insulator wafers. Microring resonators are label-free sensors that support spectrally well-defined optical modes that are extremely sensitive to surface binding-induced changes in the local refractive index. Each microring can be uniquely functionalized with a biomolecularly-specific capture agent (antibody, cDNA, aptamer, etc.) allowing sensitive and multiplexed detection of a range of protein and nucleic acid targets at sub-pM levels and from within complex matrices, such as human serum. This paper describes recent advances in sensor design, characterization, and detection of biomolecules for applications in bioterrorism surveillance and medical diagnostics, with an added emphasis on the incorporation of emerging capture agents that give improved sensor performance.