Bid is an abundant pro-apoptotic protein of the Bcl-2 family that is crucial for death receptor-mediated apoptosis in many cell systems. Bid action has been proposed to involve the mitochondrial re-location of its truncated form, tBid, to facilitate the release of apoptogenic proteins like cytochrome c. However, the precise mechanism of (t)Bid action is unknown. To advance our knowledge, this review evaluates the basic steps of Bid activation--caspase cleavage, dissociation of tBid, and lipid-mediated mitochondrial relocation--and their structure-function aspects. Relevant current information is thoroughly examined to outline the problems that hamper our understanding of the possible roles of Bid in cell life and death, and suggest valuable directions for obtaining a clarification of its pro-apoptotic mechanism.