Electrochemical sensing of biomolecules eliminates the need for the bulky and expensive optical instrumentation required in traditional fluorescence-based sensing assays. Integration of the sensor interface electrodes and active electrochemical detection circuitry on a CMOS substrate miniaturizes the sensing platform, enhancing its portability for use in point-of-care applications, while enabling the high-throughput, highly parallel analysis characteristic of traditional microarrays. This paper describes the design of a four-by-four active sensor array for multiplexed electrochemical biomolecular detection in a standard 0.25- m CMOS process. Integrated potentiostats, comprised of control amplifiers and dual-slope analog-to-digital converters, stimulate the electrochemical cell and detect the current flowing through the on-chip gold electrodes at each sensor site that results from biomolecular reactions occurring on the chip surface. Post-processing steps needed to fabricate a biologically-compatible surface-electrode array in CMOS that can withstand operation in a harsh electrochemical environment are also described. Experimental results demonstrating the proper operation of the active CMOS array for biomolecular detection include cyclic voltammetry of a reversible redox species, DNA probe density characterization, as well as quantitative and specific DNA hybridization detection.