Phase transitions are a familiar phenomenon in non-living matter, but our appreciation of their importance in biological systems has exploded in the last few years. Multiple lines of evidence now indicate that many biomolecules participate in liquid-liquid phase separation under physiological conditions and that this phenomenon helps regulate essential cellular functions such as embryonic development, ribosome biogenesis, transcription, and translation. Misregulation of phase separation has also been strongly implicated in diseases of aggregation such as Alzheimer’s and ALS, underscoring the importance of understanding the physical and chemical interactions that govern intracellular phase separation. One important clue comes from the observation that nucleic acids (DNA and RNA), and the proteins that bind them are involved in many phase separations. In addition to carrying information in their sequence, nucleic acids are highly-charged polyanions, suggesting that charge-charge interactions may promote phase separation through a process known as complex coacervation. I will discuss results of our studies of this phenomenon in vitro using oligonucleotides and cationic peptides as model systems; we find that, while nucleic acid phase separation shares many features with complex coacervation of abiotic polymers, nucleic acids’ ability to hybridize into double-stranded structures has profound effects on their phase behavior. In particular, the degree of double-strandedness of a nucleic acid determines whether a solid or liquid is formed when it is mixed with cationic polymers, and that liquid-solid phase transitions can be programmed by addition of complementary sequences or chemical denaturants. I will also discuss our efforts to put these same interactions to work at the nano-scale by engineering polyelectrolyte complex micelle nanoparticles for therapeutic delivery of nucleic acids. Multi-modal experiments, including small-angle X-ray scattering data from the APS, have enabled us to develop design rules for oligonucleotide/cationic block copolymer nanoparticles with desired size and shape.