The utilization of single-walled carbon nanotubes (SWNTs) in large quantities for electronics, optoelectronics, biosensors, and medical applications will require SWNTs of the same physical structure, electronic type, and band gap. Since current methods of synthesis produce SWNTs with polydisperse structure and properties, post-production purification is highly desirable. Here, we demonstrate a scalable method for enriching SWNTs by diameter and electronic structure using density gradient centrifugation. Importantly, this technique does not require covalent modification of the SWNTs, which can significantly degrade their outstanding electronic, optical, and mechanical properties. Furthermore, density gradient centrifugation is compatible with SWNTs of arbitrary length. To date, we have demonstrated purification in aqueous density gradients using a variety of noncovalent biofunctionalizations including sodium cholate, sodium dodecyl sulfate, and DNA. Atomic force microscopy analysis confirms enrichment by diameter and reveals SWNTs of various lengths between 0.1 and 1 micron. Furthermore, optical absorption and photoluminescence spectroscopy verify improvements in the monodispersity of SWNT optical properties. Finally, thin film field effect transistors (FETs) have been fabricated using SWNTs that are enriched by metal or semiconducting electronic type. The purified semiconducting SWNT thin films yield FET on/off ratios that are 10,000 times higher than their metallic counterparts. Overall, this non-destructive and scalable separation strategy is expected to impact a variety of applications for SWNTs where monodisperse structure and electronic properties are essential.