The present invention provides a compound comprising the structure shown in Formula I for use in delivering a therapeutic nucleic acid, one or more drug agents, or a biologically active agent to a cell, R3-(C=O)-Xaa-Z-R4 Formula IwhereinXaa is histidine, 1-methylhistidine, pyridylalanine, lysine, arginine, homoarginine, nor-norarginine, ornithine, homolysine, asparagine, N ethylasparagine, glutamine, 4-aminophenylalanine, and nor-arginine, or an N-methylated version thereof; R3 is independently a lipophilic tail derived from a naturally-occurring or synthetic lipid, phospholipid, glycolipid, triacylglycerol, glycerophospholipid, sphingolipid, ceramide, sphingomyelin, cerebroside, or ganglioside, wherein the tail may contain a steroid; or a substituted or unsubstituted C(3-22)alkyl, C(6-12)cycloalkyl, C(6-12)cycloalkyl-C(3-22)alkyl, C(3-22)alkenyl, C(3-22)alkynyl, C(3-22)alkoxy, or C(6-12)alkoxy-C(3-22)alkyl;R4 is independently a lipophilic tail derived from a naturally-occurring or synthetic lipid, phospholipid, glycolipid, triacylglycerol, glycerophospholipid, sphingolipid, ceramide, sphingomyelin, cerebroside, or ganglioside, wherein the tail may contain a steroid; or a substituted or unsubstituted C(3-22)alkyl, C(6-12)cycloalkyl, C(6-12)cycloalkyl-C(3-22)alkyl, C(3-22)alkenyl, C(3-22)alkynyl, C(3-22)alkoxy, or C(6-12)alkoxy-C(3-22)alkyl;wherein both R3 and R4 are lipophilic tails; andZ is NH, O, S, -CH2S-, -CH2S(O)-, or an organic linker consisting of 1-40 atoms selected from hydrogen, carbon, oxygen, nitrogen, and sulfur atoms;and salts thereof.