The present disclosure relates to ionisable lipids of Formula I and related structures that comprise a headgroup, core, and hydrophobic substituents. The lipids are capable of forming lipid particles, including lipid nanoparticles (LNPs), in combination with additional lipids such as neutral lipids, charged lipids, structural lipids, or PEGylated lipids. The lipid particles are suitable for delivery of therapeutic agents, including polynucleotides, peptides, antibodies, and small molecules. Compositions comprising the ionisable lipids and methods of forming and using the lipid particles are also provided.