In Chapter 2, we classify the 24 justification logics by introducing an embedding relation on them that extends that of Fitting [Fit08]. This embedding gives rise to an equivalence relation, which is natural in the sense that two equivalent justification logics realize the same modal logic. The modular realization theorem in Chapter 3 will also establish the converse direction of this statement, thereby demonstrating that the embedding provides the right level of granularity among justification logics. The machinery of embeddings also enables us to study minimal sets of operations on terms. For instance, we show that some logics do I am also especially thankful to Roman Kuznets. I learned a lot from him and working with him was always a great pleasure. Many ideas and results presented in this thesis evolved from the collaboration with him. Special thanks also go to Kai Brünnler for a successful and fun collaboration and for patiently answering my questions on proof theory.