A generalized mathematical model has been developed for predicting the strength of spun yarns produced by ring, rotor, air-jet and open-end friction spinning systems. The model is based on the translation of fiber bundle tenacity into the yarn tenacity and it is applicable to all spun yarns. The model describes the failure behavior of spun yarns. Experimental strength results are compared with the predicted ones on the basis of theory, and a satisfactory agreement is reached. The limitations of the model are also analyzed.