Various lipid nanoparticles are disclosed, including nanoparticles comprising a lipid bilayer comprising a phospholipid, a sterol, a polyethylene glycol-lipid surrounding an aqueous core which comprises a therapeutic and/or diagnostic agent and nanoparticles comprising a lipid monolayer surrounding a hydrophobic core. Of particular interest are limit size lipid nanoparticles with a diameter from 10-100 nm. Such lipid nanoparticles are the smallest particles possible for a specific particle composition. Methods and apparatus for preparing such limit size lipid nanoparticles are disclosed.