A novel transmission line reflection and delay-based ID generation scheme for Radio Frequency Identification (RFID) and other applications is presented. The proposed scheme, after receiving a sinusoidal interrogation input pulse, constructs an OOK modulated composite signal pattern that represents a unique ID code. The circuit considered consists of open or ground terminated multiple transmission lines of different lengths, and utilises reflection due to impedance mismatch and length dependent propagation delay to construct OOK modulated signal patterns. Examples of dual line and step impedance line ID circuits considering three reflected pulses in six bit positions are presented. The proposed scheme also allows larger number of bits to be generated. The ID generation scheme reported can be integrated with planar antenna to achieve planar passive RFID tags.