Publisher Summary This chapter summarizes the results of investigations of the biological activities of oligonucleotides and polynucleotides that can be used for the development of therapeutics. Specific interactions of oligonucleotides with different nucleic acid-related proteins suggest the design of oligonucleotide-based inhibitors for different regulatory factors and nucleic acid polymerases. Recent studies have demonstrated that oligonucleotides can interact with a broad spectrum of proteins capable of binding polyanions. Natural oligonucleotides do not meet the criteria of potential therapeutics. To achieve the goals of using oligonucleotide derivatives as biologically active compounds, approaches for the synthesis of different oligonucleotide analogs and conjugates have been developed. The main types of oligonucleotide derivatives and the approaches for targeting RNA and DNA are reviewed in the chapter. Drug development using oligonucleotide-based technologies represents a revolutionary strategy that directly targets and inhibits gene expression. The opportunities to develop oligonucleotides with novel properties have increased dramatically because of the recent successes in the design of novel oligonucleotide analogs and the elaboration of molecular selection approaches.