The acquisition of dissolved inorganic carbon by aquatic primary producers became increasingly challenging with higher structural complexity of algae, and with simultaneously declining atmospheric CO2 partial pressure. The seemingly easy diffusive supply of CO2 to RubisCO turned into a bottleneck for photosynthesis, which consequently required alternative inorganic carbon acquisition processes and pathways to evolve. In order to ensure sufficient CO2 supply to RubisCO, aquatic photosynthesizing organisms started to employ facilitated CO2 uptake, active HCO3- trafficking across multiple membranes as well as carbonic anhydrases, located at the outer cell membrane and in several cellular compartments. The modes of these so-called CO2-concentrating mechanisms (CCMs) are very diverse, non-canonical even within phylogenetic groups, and possess differently efficient CO2 accumulation capacities, depending on the requirements of RubisCO, the physico-chemical conditions in the boundary layer, membrane properties and cellular architecture. However, different independently evolved CCMs also exhibit a high degree of functional similarity, owing to the functional similarity of the photosynthetic process. To introduce the topic to the reader, this chapter starts with a brief outline of RubisCO´s properties and the reasons why CCMs are required (4.2). Then, the principle chemical nature of dissolved inorganic carbon in water is described (4.3): Its speciation and kinetic behavior and relevant co-determinants of carbonate chemistry. We furthermore touch upon the physico-chemical basis of carbon availability in aquatic environments (4.4.), and subsequently elaborate on the known transport modes of different inorganic carbon species. Subsequently, the current state of knowledge on existing strategies in main algal groups is presented (4.5-4.9). Finally, we consider the operation of CCMs in the context of co-occurring cellular processes (4.10), such as calcification and N2 fixation, which rely on the provision of ample inorganic carbon and/or energy and, in the case of calcification, can have important consequences for compartmental pH homeostasis.