The Gordon Research Conference on Molecular and Ionic Clusters focuses on the study of small to very large aggregates of atoms and molecules. Clusters represent a fascinating class of objects and are addressed by a strongly interdisciplinary community of experimentalists and theorists. They serve as a bridge between single atoms or molecules and bulk matter, and cluster research advances our understanding of how molecular interactions and emergent phenomena evolve as atoms or molecules aggregate. Because of their large surface-to-volume ratio and structural and electronic quantum confinement effects, clusters do not just show a smooth evolution of their properties with increasing size. Instead, they often display unique size-dependent properties that greatly differ from those of both their individual constituents as well as bulk matter. Hence, clusters can in many ways be understood as a unique state of matter. In addition to their exciting and unique size-dependent structural, electronic, optical, and chemical properties, clusters afford detailed molecular-level insight into some of the most fundamental phenomena and processes encountered in physical chemistry and molecular physics. As such, cluster research is strongly interdisciplinary and highly relevant in fields as diverse as biophysics, materials science, soft matter research and catalysis. Sessions in this GRC highlight recent state-of-the-art work in the use of clusters to study chiral interactions, non-covalent interactions of biomolecules, and microsolvation processes of neutral and charged molecules, with one session devoted to dynamical phenomena. Further sessions focus on atmospherically relevant clusters ranging from small aggregates to aerosols and explore the limit of giant clusters and liquid beams, and one session will address special properties of metal containing and inorganic clusters. Experimental techniques to study clusters include sophisticated methods ranging from molecular beams and cryogenic ion traps to ultracold helium droplets and liquid jets, mass spectrometry, scattering, and spectroscopic techniques in the microwave to the XUV range. The abilities to vary the size, composition, degree of solvation, charge state, temperature, and quantum state of clusters and to obtain molecular-level information on complex systems make cluster research ideal for comparing benchmark experimental data with various theoretical approaches. Thus, the interaction of theory and experiment has been particularly fruitful in this field. Consequently, computational and theoretical work will be featured throughout the conference, including a session on recent progress in the theoretical description of clusters. The presentations at this GRC will feature invited talks, contributed oral presentations on late-breaking topics and poster sessions.