This chapter deals with applications of thallium-based catalysts in organic transformations. The first part introduces thallium(iii) and thallium(i) compounds as homogeneous catalysts. The use of thallium(iii) salts in α-aminonitrile production and in acylation reactions can be highlighted. Similarly, the use of triorganothallium compounds in carbon–carbon bond formations is also an important application. Bases containing thallium(i) (hydroxide, ethoxide and carbonate) can accelerate Suzuki crosscoupling reactions. In addition, the thallium(i) cation can be employed as an acid catalyst to promote esterification, electrophilic bromination and oxidation of alkyl aromatic compounds by oxygen/air mixtures. The second part of this chapter presents examples of applications of heterogeneous catalysts containing thallium.