Recent experimental and theoretical work has provided new insights into the intermolecular forces and mechanisms involved in the self-assembly of biological structures such as protein complexes, vesicle aggregates and structured biological materials. It appears that many biological structures can only be formed sequentially in space and time, whereby different interactions and processes occur in different regions of space and at different times. The final structure or 'state' is therefore not the thermodynamically equilibrium state, but a 'transient' structure that nevertheless performs its allotted function at optimum efficiency - be it the opening of an adhesion site or conduction pore (with a lifetime of microseconds or milliseconds), a site-specific binding protein (with a lifetime of milliseconds or seconds), a drug-delivery carrier (with a lifetime of days), or a biomaterial with a lifetime of years. Examples will be given to illustrate the generality of such systems, and the implications of non-equilibrium and transient effects to self-assembly in vivo and in vitro will be discussed.