Torque　is　one　of　the　mighty　core　parameters　of　rotating　machinery,　and　it　has　profound　implications　for　the　success　of　many　engineering　and　industrial　manufacturing　activities.　To　date,　more　and　more　torque　sensors　are　being　researched　and　manufactured　to　meet　the　requirements　of　torque　measurements　in　all　of　the　engineering　sectors　that　rely　on　accurate　measurement　of　torque　parameters　for　their　success.　A　direct　implication　of　these　constantly　growing　developments　is　the　recent　development　of　various　torque　sensors,　which　operate　based　on　more　than　ten　different　principles.　Although　several　measurement　principles　have　been　created,　torque　sensors　themselves　can　only　be　divided　into　two　categories　based　on　whether　they　come　into　contact　with　the　rotating　objects　being　measured　(contact　sensors)　or　do　not　have　any　point　of　contact　with　these　objects　(contactless　sensors).　In　this　paper,　the　concept　of　contactless　sensors　for　torque　measurement　is　redefined　and　the　major　classes　of　torque　sensors　(i.e.,　contactless　sensors　based　on　the　laser　Doppler　effect,　laser　speckle　effect,　and　optical　coherence　vibrometer,　etc.)　are　introduced　outlining　the　main　progress,　challenges,　and　future　directions　involving　torque　measurement　applications.　As　the　key　to　the　success　of　torque　measurement　systems,　some　advanced　signal　processing　techniques　that　are　commonly　used　to　enhance　the　accuracy　of　the　torque　measurement　data　are　also　introduced.