In this paper, we introduce the plurigenera, Kodaira dimensions and more generally Iitaka dimensions on compact almost complex manifolds. These are equal to the original notions when the almost complex structure is integrable. The definitions are based on the Hodge theory on almost complex manifolds. An alternative interpretation using pseudoholomorphic maps leads to fundamental geometric consequences.We show that the plurigenera and the Kodaira dimension as well as the irregularity are birational invariants in almost complex category, at least in dimension 4, where a birational morphism is defined to be a degree one pseudoholomorphic map. However, they are no longer deformation invariants, even in dimension 4 or under tameness assumption. For tamed almost complex 4-manifolds, our Kodaira dimension is bounded above by the symplectic Kodaira dimension.By studying pluricanonical maps, we are able to show the almost complex structures that achieve the top Kodaira dimension are integrable. On the other hand, the Kodaira dimension could take all the other possible integer values for non-integrable almost complex structures.When we apply our invariants to the standard almost complex structure on the six sphere S 6 , we show the Kodaira dimension is 0 and all the plurigenera are 1, which is different from the data of a hypothetical complex structure.In the appendix, we show the uniqueness of subvarieties in exceptional curve classes for irrational symplectic 4-manifolds, which was a question of the second author.