Diverse　and　wide-range　applications　of　integrated　circuits　(ICs)　and　the　development　of　Cyber　Physical　System　(CPS),　more　and　more　third-party　manufacturers　are　involved　in　the　manufacturing　of　ICs.　Unfortunately,　like　software,　hardware　can　also　be　subjected　to　malicious　attacks.　Untrusted　outsourced　manufacturing　tools　and　intellectual　property　(IP)　cores　may　bring　enormous　risks　from　highly　integrated.　Attributed　to　this　manufacturing　model,　the　malicious　circuits　(known　as　Hardware　Trojans,　HTs)　can　be　implanted　during　the　most　designing　and　manufacturing　stages　of　the　ICs,　causing　a　change　of　functionality,　leakage　of　information,　even　a　denial　of　services　(DoS),　and　so　on.　In　this　paper,　a　survey　of　HTs　is　presented,　which　shows　the　threatens　of　chips,　and　the　state-of-the-art　preventing　and　detecting　techniques.　Starting　from　the　introduction　of　HT　structures,　the　recent　researches　in　the　academic　community　about　HTs　is　compiled　and　comprehensive　classification　of　HTs　is　proposed.　The　state-of-the-art　HT　protection　techniques　with　their　advantages　and　disadvantages　are　further　analyzed.　Finally,　the　development　trends　in　hardware　security　are　highlighted.　©　2020　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.