After a brief overview of existing models in software reliability in Sect. 27.1, Sect. 27.2 non-homogeneous Poisson process (NHPP) discusses a generalized nonhomogeneous Poisson process model that can be used to derive most existing models in the software reliability literature. Section 27.3 describes a generalized generalized random field environment random field environment (RFE) model incorporating both the testing phase and operating phase in the software development cycle for estimating the reliability of software systems in the field. In contrast to some existing models that assume the same software failure rate for the software testing and field operation environments, this generalized model considers the random environmental effects on software reliability. Based on the generalized RFE model, Sect. 27.4 describes two specific RFE reliability models, the γ-RFE and β-RFE models, for predicting software reliability in field environments. Section 27.4 illustrates the models softwarefailure data using telecommunication software failure data. Some further research considerations based on the generalized software reliability model are also discussed.