Aseptic loosening of total knee arthroplasty (TKA) components is one of the frequent reasons for early revision together with infection and instability. Aseptic loosening is usually preceded by the observation of radiolucent lines (RLL) on radiographs. Radiolucent lines have conventionally been considered a sign of osteolysis due to particles disease of either polyethylene or cement wear. However, RLL can be observed quite early after TKA, way before wear and osteolysis can even occur. Immediate postoperative RLL are secondary to surgical technique with either inadequate cement penetration in sclerotic bone, insufficient preparation of the bone or malpositioning of the component relative to the bone cuts. This type of RLL can be observed radiologically but remains often without clinical symptoms. Early development of RLL, on an initially satisfying radiograph, is secondary to changes to the cement-bone interface. These are most often related to micromotion because of constraint, malalignment, remaining mechanical deformity, erroneous bone cuts or osteoporosis. This type of RLL are observed progressively on follow-up radiographs and can be accompanied by pain complaints despite of initial good outcome. Young age, male sex or osteoporotic bones often found in elderly females, are all risk factors. A special form of aseptic loosening is tibial debonding that has been observed for different types of implants and different types of cement. It occurs at the cement-implant interface with cement remaining well attached to the trabecular bone. Probably it is a lack of cement adhesion between the high viscosity cement and the component. Revision is proposed upon diagnosis to avoid component's displacement with secondary destruction of the proximal tibial bone. Finally, RLL can develop over time secondary to polyethylene wear. These lines appear because of osteolysis and bone loss and will lead at the end to aseptic loosening of the components. Symptoms are related to failure of the implant-bone construct. Radiolucent lines without clinical symptoms should be analysed according to their potential reason of development and followed up closely with adequate radiological techniques. If symptoms develop or radiological imaging objectivizes failure and component mobility, revision knee arthroplasty might be necessary.