Time-to-Digital Converters (TDCs) are key building blocks in time-based mixed-signal systems, used for the digitization of analog signals in time domain. A short survey on state-of-the-art TDCs is given. In order to realize a TDC with picoseconds time resolution as well as multi MGy gamma-dose radiation tolerance, a third-order time-domain ΔΣ TDC structure is proposed. The first prototyping TDC, implemented in 0.13 μm, consumes only 1.7 mW from a 1.2 V supply. It achieves a time resolution of 5.6 ps and an ENOB of 11 bits, when the oversampling ratio (OSR) is 250. The SNDR is mainly limited by the skew error introduced by the comparator delay, which can be mitigated by using a delay-line assisted calibration technique. It improves the ENOB of the TDC to 13 bits and achieves a wide input dynamic range of 100-ns. The TDC also exhibits enhanced radiation tolerance owing to the mismatch-insensitive nature of the ΔΣ structure. Even after a total dose of 3.4 MGy at a high dose rate of 30 kGy/h, the ENOB only drops by 1 bit and, for an OSR of 250, a 10.5 ps time resolution is still achieved.