Software Reliability Engineering (SRE) provides a way to quantify the likelihood of software failures in software-intensive systems during test and in-field operations. One simple-to-use, yet practical technique uses software reliability growth models, field and laboratory usage models, and acceleration factors to estimate software field failure rates. A case study illustrates the technique for fighter aircraft avionics software and compares predicted to observed software field failures. Establishing a target software reliability objective during software development and working toward it provides assurance that the software is achieving an acceptable operational performance mark – a mark that can be predicted, observed, and measured both in the laboratory and in the field.