The development of copying behaviours of a group of infants from 9 to 15 months of age is analysed. These behaviours are classified according to their type (mimetic or imitated) and function (instrumental, social or hybrid). The function of the behaviours is derived from observable indicators linked to dyadic interaction. Mimetic behaviours are maintained from 9 to 15 months, while imitative behaviours increase in frequency. The elicited copying situation promotes more behaviours with instrumental function, while other functions appear spontaneously. The concurrent and prospective relationships between copying behaviours -- frequency, type and function- and receptive and expressive language are different through development. This study proposes an innovative definition of the functions of copying behaviours that is based on dynamic variables linked to interaction, in contrast to previous work that derives function from static variables such as context, object use and/or the body part performing the movement.