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Green infrastructure preserves, enhances, or restores elements of a natural system to provide higher-quality, more resilient, and lower-cost infrastructure services. Ecosystem disaster prevention and mitigation is also one of the features of green infrastructure. It is thus expected to integrate gray and green infrastructure to achieve greater resilience and sustainability. While several concepts of integrated green and gray infrastructure (IGGI), or hybrid infrastructure, have been proposed at the landscape scale, hybrid possibilities could exist at the single structural element and material scales. This paper proposes three types of IGGI concepts that could be beneficial for gray engineers to implement green infrastructure in terms of the gray engineering perspectives (safety, serviceability, and durability), primarily focusing mainly on the scale of single- or multiple-structure scales smaller than landscape scale. By categorizing the past and feasible IGGI cases into three concepts, the performance requirements for each infrastructure (green or gray) will become apparent, accelerating the discussion to develop a performance curve model as an IGGI. Also, the work needed to realize IGGI is discussed, emphasizing the urgent need for gray engineers to understand the benefits of integrating green infrastructure into gray infrastructure and thus promote the implementation of IGGI. [ABSTRACT FROM AUTHOR]