The major hurdle in the development of transdermal route as a versatile drug delivery method is the formidable transport barrier provided by the stratum corneum. Despite decades of research to overcome the stratum corneum barrier, limited success has been achieved. The objectives of this research were to develop and characterize two different strategies to overcome the stratum corneum barrier for transdermal delivery of biopharmaceuticals and vaccines. In the first strategy, coated microneedles (sharp-tipped, micron-sized structures) were developed to enable delivery of drugs directly into the skin by bypassing the stratum corneum barrier. In the second strategy, instead of bypassing the barrier, microdermabrasion was used to selectively abrade stratum corneum with sharp microparticles for topical drug application. Coated microneedles For developing painless microneedles, the first detailed study was performed to characterize the effect of microneedle geometry on pain caused by microneedle insertions in human volunteers. This study demonstrated that microneedles are significantly less painful than a 26-gage hypodermic needle and that decreasing microneedle length and numbers reduces pain. Next, the first in-depth study of microneedle coating methods and formulations was performed to (i) develop a novel micron-scale dip-coating process, (ii) test the breadth of compounds that can be coated onto microneedles, and (iii) develop a rational basis to design novel coating formulations based on the physics of dip-coating. Finally, a plasmid DNA-vaccine was coated onto microneedles to immunize mice, to provide the first evidence that microneedle-based skin immunization can generate a robust in vivo antigen-specific cytotoxic-T-lymphocyte response using similar, or lower, DNA doses on microneedles as when using the gene gun or intramuscular injection. Microdermabrasion We demonstrated for the first time that microdermabrasion in monkeys and humans can selectively, yet completely remove the stratum corneum layer. Using a mobile mode of microdermabrasion, an increase in the number of treatment passes led to greater tissue removal. Furthermore, topical application of Modified Vaccinia Ankara virus after microdermabrasion induced virus-specific antibodies in monkeys. In conclusion, both coated microneedles and microdermabrasion were developed to enable delivery of biomolecules into the skin, indicating their potential for transdermal delivery of a wide range of biopharmaceuticals and vaccines.