Dust grains are ubiquitous in all astrophysical environments, from the Solar System and protoplanetary disks to interstellar and intergalatic clouds, and their influence on the radiative properties of all these very diverse media is always significant through the absorption, scattering, and (non-)thermal re-emission of starlight. They are also a major player in the determination of the interstellar gas temperature through photo-electric emission or gas-grain collisions. Similarly grains have a great influence on the chemical complexity in the interstellar medium: indeed, the role of grain-surface reactions is crucial to understand the formation of some very common molecules, such as H2, and of more complex molecules. The grain radiative properties and their catalytic efficiency are, at least, reliant on the grain size distribution, structure, and chemical composition, which vary throughout the dust lifecycle. Observations show that grain growth arises in dense molecular clouds and protoplanetary disks as traced by an enhancement of the dust far-IR emissivity, a change in the far-IR SED spectral index, and by the effects of cloud-/core-shine from the visible to the mid-IR. There are also more and more evidences for dust variations in the diffuse ISM both from cloud-to-cloud and within clouds. In the context of THEMIS (The Heterogeneous Evolution dust Model of Interstellar Solids), a core-mantle dust model, I will show how most of the variations in the observations of both diffuse and dense clouds are consistent with accretion and coagulation processes.