Metamaterials – and their two-dimensional analogues named metasurfaces – are engineered using 'building blocks' at scales beyond atomistic and macromolecular levels to achieve properties not found in their bulk constituent materials. These advanced materials have applications spanning electromagnetic, acoustic, thermal, and mechanical domains. As of 2023, the market size for metamaterials was valued at US$ 815 million, with projections indicating growth to US$ 6,442 million by 2032, driven by a compound annual growth rate of 24.5% from 2024 to 2032 onwards [https://www.imarcgroup.com/metamaterials-market]. Among the various types, mechanical metamaterials are distinguished by their exceptional mechanical properties, which enable highly customized behaviours [https://doi.org/10.1038/s41467-023-41679-8]. Polymer-based mechanical metamaterials are engineered by assembling various microstructural units which can be classified into types such as origami, chiral, and lattice-based types, depending on their microstructural design. A range of engineered polymers has been tested to create these 'building blocks', selected to impart unique or multiple mechanical properties at larger scales. These polymers include: •Elastomers like polydimethylsiloxane (PDMS) and thermoplastic polyurethanes (TPU), highly valued for their hyperelasticity, flexibility, and energy-conservation. When incorporated into structures designed to exhibit negative stiffness – where the loading force decreases as deformation increases – they enable behaviours such as self-locking and multi-stability, allowing the metamaterial to maintain deformations and recover without plastic deformation, making it effective for shock isolation and vibration control. •Thermoplastics, such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) and polycarbonate (PC), offering adjustable stiffness and strength, which are critical for developing lightweight, high-performance structures with enhanced energy absorption and impact resistance. •Thermosets, including epoxy resins and polyurethanes – praised for their rigidity, durability, and ability to maintain structural integrity under stress – or phenolic resins and cyanate esters offering thermal stability and high mechanical strength. In sandwich structures, thermosets often serve as matrix materials for auxetic cores, enhancing energy absorption and impact resistance. •More functional polymers including and not limited to (i) shape memory polymers (SMPs), which enable the creation of adaptive and responsive structures; (ii) conductive polymers, like PEDOT, which merge electrical conductivity with mechanical durability; and (iii) magnetorheological elastomers (MREs), which integrate materials like silicone and natural rubber with embedded magnetic particles to achieve tunable stiffness and damping properties under varying magnetic fields. Polymer-based mechanical metamaterials are characterized by properties such as ultra-lightweight, ultra-stiffness, negative responses (e.g., negative Poisson’s ratio, negative stiffness), and programmable responses. These unique properties enable applications in fields where conventional materials fall short, such as ultra-stiff yet lightweight structures for aerospace or materials with programmable mechanical properties for biomedical applications. Notable examples of mechanical metamaterials include: •Auxetic metamaterials exhibiting a negative Poisson’s ratio, causing counterintuitive deformation thanks to re-entrant honeycomb unit-cell geometries, thus enhancing toughness and energy absorption. •Bouligand structures, with a multi-scale helical out-of-plane arrangement of unidirectional layers of unidirectional deposited polymeric joints, known for enhanced stiffness via crack deviation mechanisms. •More complex structural designs achieved based on computer-generated topological structures which require advanced processing techniques. Several industrial demonstrators are under development, including vehicle frontal structures and impact absorbers in seats, showcasing the practical applications of these innovative materials. Similarly, mechanical metasurfaces specifically designed to control mechanical waves can dynamically alter mechanical forces and regulate vibrations. Besides, bio-inspired methodologies, which mimic natural systems to develop advanced materials with unique characteristics, are often pivotal in advancing mechanical metamaterials. Despite their promise, significant challenges persist, particularly in design, analysis, fabrication, and application. Design challenges include the uncertainty in predicting performance from structural changes and the complexity of creating the necessary microstructures. For instance, achieving lightweight structures often requires aperiodic or gradually changing microstructures rather than simple periodic unit cells. Analysis challenges involve reconciling structural assumptions with real-world performance imperfections, and addressing long-term behaviour as most characterizations still focus on monocyclic tests, neglecting fatigue performance, which is essential for understanding thermomechanical behaviour under cyclic loading conditions. Furthermore, the transition to micro and submicron scales introduces additional challenges in manufacturing precision and scalability, as current fabrication methods may not suffice for the necessary accuracy and defect-free production on these smaller scales. Fabrication challenges arise from the complexity of the required geometries, often exceeding current manufacturing capabilities to produce intricate structures accurately and without defects, even if 3D printing has recently unlocked a number of issues. Application challenges are evidenced by the limited number of patents and practical implementations, reflecting the early stage of development in this field. Addressing these gaps and challenges will be crucial for advancing mechanical metamaterials and for realising their full potential in enhancing efficiency, safety, and performance. So far, research on mechanical metamaterials has mainly focused on passive mechanical metamaterials and the tunability of their mechanical properties. Looking ahead, the miniaturization of these materials to micron and submicron scales could expand their applicability in fields like nano-robotics, microelectromechanical systems, and next-generation sensors. Additionally, deep integration of multifunctionality (sensing, energy harvesting, electrical actuation, adaptation, computation, information processing), and advancing data-driven designs could lead to truly intelligent mechanical metamaterials.