The effect of sp³-defects on the electronic and transport properties of semiconducting carbon nanotubes has been systematically studied on the basis of a quantum mechanical tight-binding model. We have calculated the band structure for carbon nanotubes with ordered defect patterns showing a large impact on the bandgap energy whereas for randomly distributed defects the band structure remains relatively robust. The transport behavior has been studied on the basis of the Green’s function method. The results indicate that the conductance of defective carbon nanotubes strongly depends on the number of defects and the tube diameter. We further show that the transport properties can be classified, depending on the number of defects, into two regimes which are either characterized by the mean-free path or the localization length. For both, analytical equations describing the impact of the tube diameter as well as the number of defects are derived. Comparing these values with the channel length indicates the dominant transport regime.