The elastic instability phenomenon plays an important role in pattern formations in soft biological systems. The phenomenon also has been actively used to design new (meta-) materials with switchable microstructures, properties, and functions [1]. Here, we investigate the elastic instability phenomenon in soft heterogeneous materials. These deformable composites typically combine soft deformable matrix and stiffer phases (such as fibers or inclusions). We report the experimental observations of wavy patterns forming in (a) soft 3D printed laminates [2] and (b) 3D-fiber composites [3], (c) twining patterns in particulate composites [4, 7, 8] and laminates [5], and (d) auxetic microstructure transformations in inclusion-matrix-void soft systems [6]. REFERENCES [1] J. Li et al. Elastic instabilities, microstructure transformations, and pattern formations in soft materials, Curr. Opinion in Solid State and Mater. Science 25, 100898 (2021) [2] V. Slesarenko. and S. Rudykh. Harnessing viscoelasticity and instabilities for tuning wavy patterns in soft layered composites. Soft Matter 12, 3677-3682 (2016) [3] J. Li, et al. Instabilities and pattern formations in 3D-printed deformable fiber composites. Compos Part B Eng., 148:114-122 (2018) [4] J. Li, et al. Domain Formations and Pattern Transitions via Instabilities in Soft Heterogeneous Materials. Adv Mater., 31, 1807309 (2019). [5] J. Li, et al. Emergence of instability-driven domains in soft stratified materials. npj Computational Materials 8, 100 (2022). [6] J. Li, et al. Auxetic multiphase soft composite material design through instabilities with application for acoustic metamaterials. Soft Matter, 14, 6171 (2018). [7] D Chen et al. Post-buckling development in soft particulate composites. Composite Structures 322, 117337 (2023) [8] Y. Xiang et al, Towards understanding the role of viscoelasticity in microstructural buckling in soft particulate composites. Compos Part B Eng, 263 110850 (2023)