This paper deals with a problem of major concern to the knitting industry, which is fabric defects. When a defect occurs, the knitting machine has to be stopped and the fault corrected, thus resulting in time loss which is uneconomic. Eventually, the knitted fabric may be rejected if quality requirements are not met. An effective monitoring of the knitting process is required in order to avoid or detect and locate a defect and its cause as soon as possible, avoiding productivity and quality losses.In circular knitting machines the yarn input tension (Ti) can be used as a means of process control, so that defects may be prevented or quickly detected. This was found to be a valuable approach to accomplish this task, since it reflects the general behaviour of the knitting machine. A measuring system is presented, along with the results obtained. Considerations are made around the problem of automatic detection and some approaches are suggested. Finally, some conclusions are drawn from the work developed.