Optimizing Mobile Food Pantry Operations Under Demand Uncertainty Managing complex systems often involves making trade-offs between different objectives. A common example is seeking fairness guarantees in sequential resource allocation problems. For example, mobile food pantries are tasked with allocating resources under demand uncertainty with the goal of simultaneously minimizing inefficiency (leftover resources) and envy (deviations in allocations). In this work, we tackle a problem established from a partnership with the Food Bank of the Southern Tier in optimizing their mobile food-pantry operations. We provide an exact characterization of the achievable (envy, efficiency) pairs, showing that any algorithm achieving low envy must suffer from poor inefficiency and vice versa. We complement this exact characterization with a simple algorithm capable of achieving any desired point along the trade-off curve. We consider the problem of dividing limited resources to individuals arriving over T rounds. Each round has a random number of individuals arrive, and individuals can be characterized by their type (i.e., preferences over the different resources). A standard notion of fairness in this setting is that an allocation simultaneously satisfy envy-freeness and efficiency. The former is an individual guarantee, requiring that each agent prefers the agent's own allocation over the allocation of any other; in contrast, efficiency is a global property, requiring that the allocations clear the available resources. For divisible resources, when the number of individuals of each type are known up front, the desiderata are simultaneously achievable for a large class of utility functions. However, in an online setting when the number of individuals of each type are only revealed round by round, no policy can guarantee these desiderata simultaneously, and hence, the best one can do is to try and allocate so as to approximately satisfy the two properties. We show that, in the online setting, the two desired properties (envy-freeness and efficiency) are in direct contention in that any algorithm achieving additive counterfactual envy-freeness up to a factor of LT necessarily suffers an efficiency loss of at least 1 / L T . We complement this uncertainty principle with a simple algorithm, Guarded-Hope, which allocates resources based on an adaptive threshold policy and is able to achieve any fairness–efficiency point on this frontier. Our results provide guarantees for fair online resource allocation with high probability for multiple resource and multiple type settings. In simulation results, our algorithm provides allocations close to the optimal fair solution in hindsight, motivating its use in practical applications as the algorithm is able to adapt to any desired fairness efficiency trade-off. Funding: This work was supported by the National Science Foundation [Grants ECCS-1847393, DMS-1839346, CCF-1948256, and CNS-1955997] and the Army Research Laboratory [Grant W911NF-17-1-0094]. Supplemental Material: The online appendix is available at https://doi.org/10.1287/opre.2022.2397. [ABSTRACT FROM AUTHOR]