Several strategies based on synthetic oligonucleotides (ON) have been proposed to control gene expression. As for most biomolecules, however, delivery has remained a major roadblock for in vivo applications. Conjugation of steric-block neutral DNA mimics, such as peptide nucleic acids (PNA) or phosphorodiamidate morpholino oligonucleotides (PMO), to cell-penetrating peptides (CPP) has recently been proposed as a new delivery strategy. It is particularly suitable for sequence-specific interference with pre-mRNA splicing, thus offering various applications in fundamental research and in therapeutics. The chemical synthesis of these CPP–ON conjugates will be described as well as easy-to-implement assays to monitor cellular uptake, endosome leakage, and efficiency of splicing redirection.