The research explores the relationship between uncertainty, flexibility and Life Cycle Design in the design of complex systems, in general, and in the particular case of the design of building systems. In the architectural-engineering works, conceived as activities aimed at generating new systems, the comparison with the uncertainty is inevitable. There is a wide range of types of uncertainty; a possible simplification is to consider two types of causes of uncertainty, the presence of the internal or external uncertainties to the system. That is the variability of demand (unknowns about the social and economic context) or the variability of technological market (unknowns about the performance of the system), in other words the functional obsolescence and the technological obsolescence. If flexibility is the ability of a system to be easily modified and to respond to changes in the environment in a timely and convenient, then the flexibility can be considered the antidote to obsolescence, or the characteristic of the system that guarantees slippage over time. It’s that property that makes the system resilient able to absorb the shock and/or disturbance without undergoing major alterations in its functional organization, in its structure and in its identity characteristics. In the paper, the flexibility is therefore seen as a fundamental property for designing a generally complex system, and particularly in architectural design, through the identification of design criteria for the implementation of this requirement, that influence on architecture (form more technology) of the system.