This chapter explores advanced modulation, coding, and signal processing techniques to enhance flexibility, spectral efficiency, and reliability in next-generation elastic optical networks. First, a non-uniform adaptive PSK modulation scheme is introduced, dynamically adjusting constellation amplitudes based on channel conditions to balance receiver complexity and service-specific Quality of Service (QoS) requirements. Building on adaptability, a 3D hierarchical QAM framework is proposed, enabling multi-dimensional constellation optimization through adaptive spacing and bit-mapping control. To further optimize spectral efficiency, a probabilistic shaping (PS) 4D modulation scheme is demonstrated in a self-homodyne coherent transmission system. Additionally, the chapter introduces a concatenated coded modulation scheme combining adaptive turbo codes with trellis-coded modulation (ATTCM) in a generalized frequency division multiplexing (GFDM) framework. This hybrid approach delivers high coding gains and spectrum efficiency with low complexity, validated experimentally in a 4 Gbit/s optical access system. We further propose a novel probabilistic shaping trellis-coded modulation (PS-TCM) scheme integrated with GFDM in an intensity-modulation/direct detection (IM/DD) system.