Definition 0.1. Groupoid categories, or categories of groupoids, can be defined simply by considering a groupoid as a category 𝒢₁ with all invertible morphisms, and objects defined by the groupoid class or set of groupoid elements; then, the groupoid category, 𝒢₂, is defined as the 2-category whose objects are 𝒢₁ categories (groupoids), and whose morphisms are functors of 𝒢₁ categories consistent with the definition of groupoid homomorphisms, or in the case of topological groupoids, consistent as well with topological groupoid homeomorphisms. The 2-category of groupoids 𝒢₂, plays a central role in the generalised, categorical Galois theory involving fundamental groupoid functors.