Apel's work brings together the analytical and Continental philosophical traditions, especially pragmatism and the critical theory of the Frankfurt School.

[4] In Understanding and Explanation: A Transcendental-Pragmatic Perspective, Apel reformulated the difference between understanding (Verstehen) and explanation (Erklärung), which originated in the hermeneutics of Wilhelm Dilthey and interpretive sociology of Max Weber, on the basis of a Peircean-inspired transcendental-pragmatic account of language.

This account of the "lifeworld" would become an element of the theory of communicative action[5] and discourse ethics, which Apel co-developed with Jürgen Habermas.

After taking his point of departure from Apel, Habermas has moved towards a "weak transcendentalism" that is more closely tied to empirical social inquiry.

An early German-speaking adversary of so-called critical rationalism, Apel published a critique of the philosophy of Karl Popper: In Transformation der Philosophie (1973), Apel charged Popper with being guilty of, amongst other things, a pragmatic contradiction.