The electrical performance of semiconducting materials can be strongly affected by point defects. In carbon nanotubes (CNTs), a wide variety of defects are possible (both chemical and electrostatic) with an equally wide variety of electronic consequences. Understanding these electronic consequences is critical for high performance CNT transistor applications, as well as CNT sensor application. I will discuss our recent measurements of three types of carbon nanotube defects: (1) oxidative point defects, (2) electrostatic barriers generated by charge traps, (3) sp3-bonded phenyl groups. The defects we study are either naturally occurring, created by scanning probe modification, or created by chemical modification of the CNT sidewall. In all cases, real-time electrical measurements are invaluable for monitoring defect creation and identifying single defects. We show that a single sp3-bonded phenyl groups has minimal effect on device properties, while certain oxidative defects dominate global device properties. We also confirm recent theoretical predictions for carrier scattering from a Coulomb barrier in a CNT, and the dependence of this scattering probability on gate voltage.