This thesis studies the relationship between actions of monoidal categories and enrichments in monoidal categories, and uses these ideas to prove the existence of universal enrichments of a category. It first details the correspondence between actions of a monoidal category 𝓥 on a category 𝓐 which are right-closed, and enrichments of 𝓐 in 𝓥 which are copowered, and explains how this restricts back to an equivalence between cocomplete enrichments of the cocomplete category 𝓐, and right-closed monoidal actions which preserve colimits in the A-variable. It then considers the locally presentable case, showing that any locally presentable category 𝓐 has an enrichment in the locally presentable monoidal category Cocts(𝓐, 𝓐) of cocontinuous endofunctors of 𝓐. It shows that this is a universal cocomplete enrichment of 𝓐, in the sense that any other cocomplete enrichment of 𝓐 is obtained from the universal one via base-change. It then considers the case of actions of braided monoidal categories 𝓥 on monoidal categories 𝓐. It shows that right-closed such actions correspond to enrichments of 𝓐 to a monoidal 𝓥-category. Again, there is a notion of universal enrichment, and the thesis concludes by sketching a proof that a locally presentable monoidal 𝓐 has a universal monoidal enrichment in its monoidal centre 𝓩(𝓐).