The interaction properties of atoms are, at low temperatures, fully determined by the s-wave scattering length of the interatomic interaction potential. The magnitude and sign of this quantity strongly depend on the presence of bound states in this potential and, more precisely, on the energy of the bound state that is closest to the continuum threshold. In the multichannel case of a Feshbach resonance, the energy of the two colliding atoms in the incoming open channel is close to the energy of a bound state, i.e., a molecular state, in a coupled closed channel. Due to the different spin arrangements of the atoms in the open channel and the atoms in the molecular state, the energy difference between the bound state and the continuum threshold is experimentally accessible by means of the Zeeman coupling of the atomic spins to a magnetic field. As a result, one is able to vary the scattering length to any possible value by tuning the magnetic field. This level of experimental control has opened the road for many beautiful experiments which recently led to the demonstration of coherence between atoms and molecules, by observing coherent oscillations between atoms and molecules, analogous to coherent oscillations that are observed in ordinary two-level systems. We review the theory that describes coherence between atoms and molecules in terms of an effective quantum field theory for Feshbach-resonant interactions. The theoretical predictions resulting from this theory are in excellent agreement with experimental results.