Decisions made in the early design stages commit the majority of the aircraft life-cycle costs. Aerospace development requires enormous investments and hence, the decision-maker needs confidence to launch a program. Uncertainty exists either by aleatory sources or lack of knowledge and must be handled somehow. The use of design margins based on previous experience is well established within industry to prevent that the design fails to comply with its requirements. However, margins based on previous developments may be inappropriate or obsolete for new products which combined to the lack of robustness/reliability measures can lead to oversized designs that are undoubtedly penalized by the ''no free lunch'' rule and which have no guarantee of feasibility under uncertainty. This work develops an Uncertainty-based Multidisciplinary Design Optimization (UMDO) framework capable of quantifying the required design margins to ensure feasibility at a given robustness level whilst taking into account uncertainties associated to current and/or predicted state-of-knowledge. A conceptual design optimization case study was used to demonstrate how aiming for higher probability of constraint satisfaction affects the design and objectives spaces, providing quantitative and visual information on the Price of Feasibility Robustness to the decision-maker who can now make risk-informed decisions. Comparison of the traditional margin setting strategies with optimization under uncertainty showed how empirical margins not only result in less competitive designs, but also in designs that do not necessarily comply with all requirements at the desired robustness level, whereas designs obtained via UMDO are optimal with respect to the chosen objective functions with ensured feasibility under uncertainty. The UMDO framework was also used to assess how input uncertainty variability affects the design and objective spaces for a given robustness target, presenting quantitative information on the Cost of Uncertainty to the decision-maker. This process becomes a powerful 'what-if' tool capable of quantifying what would be the achievable benefits in the design if a subset (or the whole set) of epistemic uncertainties were to be reduced through some knowledge augmentation mechanism. Another critical step in Conceptual Design is the selection of technologies to be incorporated into the design. The long development cycle inherent to the aerospace and defense industry requires both proven and emerging technologies to be studied for incorporation to avoid an early obsolescence of the design. An uncertainty-based framework to support technology portfolio selection while accounting for maturity metrics at system-level and uncertainty in the prediction of technology effects is proposed and applied to a Future Fighter design case study. The framework enables designers and decision-makers to conduct trade-offs between technology maturity, multiple measures of performance, and robustness.