The transient as well as the long-term behavior of the spin-density operator under the influence of radiofrequency pulses and periods of free-precession can be predicted to a reasonable degree of accuracy within the framework of the Average Liouvillian Theory. This approach can be used to design pulse sequences which suppress the effects of certain components of the Liouvillian on the spin dynamics while allowing the detailed analysis of the effects of others. We present here the basic principles of Average Liouvillian Theory and its application in the design of pulse sequences to study the structural and dynamic properties of biomolecules. © 2000 John Wiley & Sons, Inc. Concepts Magn Reson 12: 152-172, 2000