M-theory is well-known as a non-perturbative completion of string theory and as such is supposed to provide a unified quantum theory of all forces including gravity. In addition to supergravity fields, M-theory contains branes whose dynamics are governed by novel strongly coupled field theories decoupled from gravity. In this Colloquium we will discuss what M-theory implies for Quantum Field Theories, particularly those in 5 and 6 dimensions.