The concept of design for testability (DFT) for digital and analog circuits is described hereafter. After explanations of the necessity for this approach, some ad hoc ideas are presented. These are used to justify the implementation of the scan-test conceptScan-test concept which eventually becomes IEEE 11.41 standard. The implementation of this standard to digital circuit (IEEE 1141.1) is briefly reviewed first. That should allow understanding of the very concept. Then the version intended to be used for mixed-signal systems (IEEE 1141.4) is introduced. Examples are given for the implementation of IEEE 1141.4 standard. For both standards, the concept of DFT implemented at circuit schematic level is explained and exemplified.