This paper presents a comprehensive review of the progress in nuclear TH R&D activities undertaken in the Republic of Korea over the past few decades. The focus is mainly on the developments that commenced after Korea decided to embark on large-scale national projects with the aim to develop advanced reactor designs. Korea operates predominantly light water reactors, so the paper deals exclusively with LWR TH. The Fukushima Daiichi accident also has imparted an indelible impact on Korea's national nuclear program in general and TH research in particular, as the accident has forced Korea to reset the priorities and directions in subsequent R&D efforts. The paper covers many technical and scientific topics that were deemed to be of contemporary importance and interest at the time of their developments. TH has been at the front and center of Korea's nuclear program and TH research evolved through a few phases aligned with the overall national nuclear technology development program. TH research program was kindled in earnest during the nascent period of nuclear technology development coinciding with the operation of the nation's first commercial nuclear plant, Kori-1, in 1978. The embryonic research period segued into next phase when Korea decided to indigenize imported nuclear technology. National research programs began to blossom when they entered next phase, which was prompted by Korea's aspiration and desire to develop its own advanced reactor designs. This period then transitioned into the current phase, largely dictated by the Fukushima Daiichi accident. Various phases had their unique research needs of the times. The paper highlights some major or critical TH technologies developed over the years.