Following the formulation of time reversal in quantum mechanics by Eugene Wigner in 1932, the early 1950s saw the first attempts at a formulation of time reversal in quantum field theory (QFT). The variety of these attempts and the differences between them exhibits the conceptual difficulty encountered by physicists in thinking about time reversal, its role and significance in QFT, and how it is tied to other symmetry transformations. Through the combined efforts of Julian Schwinger, Gerhart Lüders, and Wolfgang Pauli between 1951 and 1955, it became clear that time reversal (T) in QFT is most naturally considered in combination with charge conjugation (C) and spatial reflection (P) and that indeed all (local relativistic) QFTs are CPT-invariant. This talk will address how the four different approaches to time reversal investigated by these authors led to the first formulations of the CPT theorem by Lüders in 1954 and Pauli in 1955.