Great differences between published Heckel parameters, obtained from 'at pressure' data or the 'in-die' method, are outlined. The general validity of the concept of yield pressures derived from slopes of such Heckel plots is questioned. When the ability of the Heckel and the Walker equations is compared to fit density/pressure data from tabletting different pharmaceutical powders, a generally better fit is obtained with the Walker equation in the region of 5-100 MPa. The ability to discriminate between materials by data from the compression phase is improved by using the Walker model. For Emcompress®, apparent yield pressures derived from Heckel plots are dependent strongly on the maximum pressure of the compression process.