The contents of this chapter discuss the relationship between paradigm, theories, and models in the existentialist perspective. For existentialism we mean a specific cultural climate in the twentieth-century old Europe as well as in the USA whose philosophical paradigm has its roots in some Greek philosophers, although the Danish thinker Søren Kierkegard and the French philosopher Jean-Paul Sartre are among its most important exponents. We will explore just three psychological approaches of this not homogeneous movement including many different schools of thought sometimes conflicting: the phenomenological perspective, the antipsychiatry movement, and existential psychotherapies. In this temporal and conceptual excursus, we will investigate whether the existentialist paradigm has generated theories and/or models that can be qualified as particular-specific or general-integrative.