Composable basically means units that are compatible so that they can be connected in any order and in any number -- "from many into one."

Assembling two component BioBricks always results in a new, larger BioBrick that can be used in future assemblies. By being composable, BioBricks may be created that become longer and more complex in function, but since the connections between the component units are all the same, the practical work of extension is identical at each step.

Composability is one of the key features of BioBrick standard assembly.