High pressure levels of strongly focused therapeutic transducers create new challenges for formulating international standards for measurement and simulation. Earlier efforts of Working Group 6, High Intensity Therapeutic Ultrasound and Focusing Transducers, Technical Committee 87 of the International Electrotechnical Commission, resulted in a standard for measuring high power (up to 500 W) and a specification for measuring ultrasound fields at lower amplitudes using existing linear methods and scaling up to higher therapeutic levels. Presently, work is underway on a new document to describe direct approaches for characterizing ultrasound pressure fields in water up to 100 MPa through the use of state of the art optical and robust hydrophones. Because the real goal of our efforts is to determine therapeutic pressure levels in tissue at a target site, we have initiated work on a second hybrid approach which combines characterization of the radiated field close to the transducer in water with nonlinear field simulation programs. In this way the characterization data which preserve the uniqueness of a particular transducer are used as input to a simulator program for realistic field prediction. Our group is addressing how to simulate nonlinear propagation in heterogeneous absorbing tissues accurately and to benchmark simulation software.