Carbon nanotubes (CNTs) can be applied as versatile biopharmaceutical delivery systems due to their high drug-loading capacity, excellent cell-penetrating ability, and customizable surface chemistry. Biopharmaceuticals are potent therapeutics when applied correctly, but are challenged by significant drawbacks. The coupling of biopharmaceuticals to CNTs can mitigate these issues, thus additively or synergistically improving their performance. Improved therapeutic effects, enhanced (targeted) delivery to specific tissue types, highly controlled release profiles, and even the visualization of in-vivo biodistribution comprise just a few examples of the benefits of such complexes. Furthermore, novel complexes have been tailored to combat cancer and other difficult-to-treat diseases, with promising success. In this chapter, the application of CNTs for the delivery of biopharmaceuticals to disease sites in vitro and in vivo will be discussed.