This review discusses the role of muscle receptors, in particular, that of muscle spindles, in the detection of movements, both passive and active. Emphasis is placed on the importance of conditioning the muscles acting at a joint before making measurements of thresholds to passive movements, to take into account muscle's thixotropic property. The detection threshold:movement velocity relation is discussed and described for a number of different joints. Implications for muscle spindles are considered from the generalisation that, when expressed in terms of proportion of muscle fascicle length change, detection thresholds are about the same at different joints. It is concluded that the available data supports the view that muscle spindles lie in parallel with only a portion of a muscle fascicle and not the whole fascicle. At the elbow joint, where it has been tested, movement detection threshold is lower during passive movements than during contraction of elbow muscles. Both peripheral mechanisms and mechanisms operating within the central nervous system may be responsible for the rise in threshold. The signalling of movements by spindles during a contraction raises the question of how the central nervous system is able to extract the length signal under such circumstances, given that there is likely to be co-activation of alpha and gamma motoneurones. The evidence for a central subtraction of fusimotor-evoked impulses and some recent experiments relevant to this idea are described. In conclusion, a number of points of uncertainly have been revealed in this area and these should be the subject of future experiments.