The integration of Unmanned Aerial Vehicles (UAVs) with wireless power transfer (WPT) technology represents a transformative advancement in modern energy delivery systems, offering unprecedented flexibility and efficiency in powering remote or hard-to-reach devices. This chapter considers a multi-UAV-assisted WPT network, where multiple UAVs are deployed to extend the lifespan of multiple ground devices (GDs) by providing WPT services. To ensure fairness in energy distribution among multiple devices, this chapter aims to maximize the harvested energy of the GD with the worst conditions through the joint optimization of UAV trajectories and transmit power design. Key factors such as UAV trajectories, collision avoidance, and power constraints are carefully accounted for in the optimization process. Diverging from traditional linear models, this chapter adopts a practical sophisticated multi-source nonlinear energy harvesting (EH) model, allowing a more precise representation of the energy conversion dynamics. Due to the highly non-convex nature of the original problem, arising from the presence of coupled variables, this chapter introduces a convex approximation method, which facilitates the transformation of the problem into a series of tightly convex subproblems. This approach enables efficient iterative solutions, leading to a high-quality solution for the joint design optimization. Finally, numerical results are provided to demonstrate the convergence of the proposed algorithm and to validate the performance benefits of the proposed multi-UAV WPT scheme with a nonlinear EH model compared to benchmark methods.