Electrochemical　energy　storage　systems　are　fundamental　to　renewable　energy　integration　and　electrified　vehicle　penetration.　Hybrid　electrochemical　energy　storage　systems　(HEESSs)　are　an　attractive　option　because　they　often　exhibit　superior　performance　over　the　independent　use　of　each　constituent　energy　storage.　This　article　provides　an　HEESS　overview　focusing　on　battery-supercapacitor　hybrids,　covering　different　aspects　in　smart　grid　and　electrified　vehicle　applications.　The　primary　goal　of　this　paper　is　to　summarize　recent　research　progress　and　stimulate　innovative　thoughts　for　HEESS　development.　To　this　end,　system　configuration,　DC/DC　converter　design　and　energy　management　strategy　development　are　covered　in　great　details.　The　state-of-the-art　methods　to　approach　these　issues　are　surveyed;　the　relationship　and　technological　details　in　between　are　also　expounded.　A　case　study　is　presented　to　demonstrate　a　framework　of　integrated　sizing　formulation　and　energy　management　strategy　synthesis.　The　results　show　that　an　HEESS　with　appropriate　sizing　and　enabling　energy　management　can　markedly　reduce　the　battery　degradation　rate　by　about　40%　only　at　an　extra　expense　of　1/8　of　the　system　cost　compared　with　battery-only　energy　storage.