In this project it was decided to investigate how different prodrugs could be used to improve the topical delivery of aciclovir (ACV) through passive and active, electrically-assisted delivery into and across different biological barriers. Chapter 1 describeds an investigation of the topical iontophoretic delivery of a series of amino acid ester prodrugs of aciclovir as a means to enhance cutaneous, and more specifically, intraepidermal delivery of ACV. Chapter 2 compares cutaneous bioavailability following topical application of marketed aciclovir formulations to that achieved after iontophoresis of ACV or its amino acid ester prodrugs. Chapter 3 describes a successful investigation into the transcorneal and transscleral iontophoretic delivery of three amino acid ester prodrugs of aciclovir and as a means to increase ocular bioavailability of ACV. Chapter 4 describes a method to enhance topical passive topical delivery of aciclovir using a series of lipophilic aciclovir prodrugs in combination with supersaturation.