With the standardization of limb-sparing surgery, megaprostheses have become one of the fundamental tools in orthopedic oncology for the reconstruction of large segmental bone and joint defects. The aim of this review is to summarize the design principles of oncological megaprostheses used in the upper and lower extremities, region-specific design options, and their reflections on the clinical decision-making process from a “past-present-future” perspective. Megaprosthesis designs have evolved around material selection, stem geometry, and fixation (cemented/uncemented, compressive osteointegration), level of joint surface limitations (fixed hinge – rotary hinge, anatomical – reverse shoulder, dual mobility, etc.), surface technologies supporting osteointegration, and soft tissue attachment solutions. Current design approaches for the proximal humerus and shoulder girdle, humeral diaphysis and total humerus, distal humerus/elbow, pelvis- periacetabular region, proximal and distal femur, proximal tibia, and total femur are presented here. The biomechanical requirements of each region, soft tissue conditions, and oncological limitations are discussed. Complications such as mechanical failure, aseptic loosening, soft tissue failure, infection, and local recurrence, as defined by the Henderson classification, and the design innovations developed in response to them (rotational hinges, HA-coated collars, silver coatings, extensor/abductor reconstruction with mesh, etc.) are discussed in detail. The success of megaprosthesis depends not solely on implant design, but on the evaluation of patient-specific factors, tumor localization, defect type, soft tissue coverage, expected survival time, and the center’s experience. New technologies such as patient-specific 3D-printed implants, bio-prosthetic hybrid reconstructions, digital planning, and virtual-augmented reality-based surgical planning are paving the way for more personalized and correctable “lifelong reconstruction” strategies in the future.