Because of its counterintuitive nature, the Negative Group Delay (NGD) remains as an uncommon and unfamiliar electronic function. For this reason, the design and analysis of NGD circuits are not well-known for most of electronic designers. This paper initiates a basic and easy to understand theory, in addition to a design methodology for the low-pass NGD function. The circuit theory on the low-pass NGD function is described using an NGD passive topology which is constituted by a RC-parallel network with a resistive load. The NGD analysis and synthesis equations in function of NGD specifications are provided and a proof-of-concept of 6-dB low-pass NGD circuits has been designed, simulated, fabricated and tested. Frequency and time domain analyses have been performed to validate the low-pass NGD function. Theoretical and simulated results are in very good agreement and an NGD has been obtained in measurement for the proposed structure