With the introduction of policies related to low-altitude economy,drone delivery, as an important driving force for enhancing new types of productivity,has gained widespread attention. Due to the limitations of drones in terms of batteries capacity and payload constraints, scholars have conducted extensive research on drone delivery models and systems. This paper aims to review the current literature in drone delivery and explore possible future research directions. This paper is conducted from three perspectives: the types of drone delivery problems,the modes of drone delivery systems,and the optimization scheduling methods for drone delivery systems. First, drone delivery problems are categorized into three types based on the services provided: pick-up only, delivery only, pick-up and delivery simultaneous, and point-to-point delivery. Second,drone delivery modes are divided into two categories based on whether drones are the sole tools for pickup and delivery:drone delivery systems and vehicle-drone collaborative delivery systems. Third, the paper introduces common optimization objectives in drone delivery problems, and reviews both single-objective and multi-objective optimization methods for solving drone delivery problems.