Quipus, knotted structures of woollen or cotton cords, were used as a bureaucratic tool in the Inca state. In the absense of a writing system, numerals and possibly other pieces of information were encoded on the quipus by tying knots into elaborately structured coloured cords. Though interpretation of the quipu contents is far from complete, some information on Inca mathematics can be deducted from the analysis of ancient specimen, especially when combined with the results of anthropological and linguistic research in contemporary Andean societies. In this paper, the quipus are introduced, their structure is explained, and some results on mathematical concepts of the Incas are presented based on a comparison of mathematical and anthropological literature on the subject.