Antisense technology constitutes development of sequence-specific DNA or RNA analogs that can block the activity of selected single-stranded genetic sequences and offer the potential of high specificity lacking in many current drug treatments. The sequencing of the human genome has greatly increased the potential of this approach. Antisense oligonucleotides, the most commonly used antisense approach, are unmodified or chemically modified single stranded RNA or DNA molecules specifically designed to hybridize to corresponding RNA by Watson-Crick binding. Phosphorodiamidate Morpholino oligomers (PMO) are a novel class of non-ionic antisense agents that inhibit gene expression by binding to RNA and sterically blocking processing or translation. PMOs have shown excellent efficiency and safety profile via various routes of administration in multiple animal and human studies. This review will summarize the preclinical studies with PMOs on the road to their development as therapeutic agents with particular emphasis on in vivo biodistribution and pharmacokinetics.