Carbon nanotube (CNT) yarns are formed by the macroscopic assembly of nanotubes well aligned along a fibre axis. Hence, due to this anisotropic assembly, CNT fibres exploit efficiently the excellent axial properties shown by nanotubes. However, fibres do not show by far the outstanding physical behaviour of individual nanotubes. This fact can be explained in terms of the complex hierarchical morphology of CNT yarns. Thus, CNTs are closely packed into bundles joined each other by Van der Waals forces; these bundles, in turn, are entangled in a high anisotropic network. Therefore, the fully transference of properties from single tubes to the macroscopic assembly is clearly diminished. In this work CNT fibres have been produced by the direct spinning method. This route consists on mechanically drawing CNT aerogel from the gaseous reaction zone of vertical CVD furnace, and then directly winding it onto a rotating rod. This is a really versatile method which enables to control the morphology of the samples by simply varying process parameters. In this regard, mechanical properties of CNT fibres obtained are in the range of high performance yarns. In fact, CNTs can be thought as rigid polymer chains. That is, both nanotubes and polymers are macromolecules consisting of long chains of repeating molecular units, and have large values of aspect ratio (length/diameter). This is why a classic analytical model used for polymer fibres has been successfully applied to this case. Through this model a close relationship between mechanical properties and morphology was successfully established. For this purpose, different morphology samples were submitted to synchrotron source X-Ray diffraction experiments in order to quantify the changes of morphology. Finally, it has been strongly reported the use of nanotubes as chemical sensors due to changes of electrical conductivity in the presence of certain chemicals. In this work it has been demonstrated the scalability of this mechanism up to fibre. In fact, a novel model based on electrochemical doping of CNT fibres has been developed. According to it, CNT yarns were successfully used as composite manufacturing s