Growth in colonial organisms by iteration of modules inherently provides for an increase in available morpho-ecospace relative to their solitary relatives. Therefore, the interpretation of the functional or evolutionary significance of complexity within groups that exhibit modular growth may need to be considered under criteria modified from those used to interpret complexity in solitary organisms. Primary modules, corresponding to individuals, are the fundamental building blocks of a colonial organism. Groups of primary modules commonly form a second-order modular unit, such as a branch, which may then be iterated to form a more complex colony. Aspects of overall colony form, along with their implications for ecology and evolution, are reflected in second-order modular (structural) units to a far greater degree than by primary modular units (zooids). A colony generated by modular growth can be classified by identifying its second-order modular (structural) unit and then by characterizing the nature and relationships of these iterated units within the colony. This approach to classifying modular growth habits provides a standardized terminology and allows for direct comparison of a suite of functionally analogous character states among taxa with specific parameters of their ecology.