This volume introduces an important concept which hitherto has not gotten the recognition it deserves. The concept simul, the idea of a both-and in theology, is primarily associated with the Lutheran understanding of justification. The formula simul iustus et peccator is a shorthand for the nucleus and heart of the Reformation. But the concept, which implies a tension or even a paradox, appears to play a significant role in other areas of theological reflection. This volume highlights a number of areas in which this idea historically has played an important role, as well as its potential in the contemporary conversation. The aim of the work is to provide an informed and readable introduction to the simul concept in various areas of systematical theology and Biblical exegesis.