This text presents a systematic discussion of three important questions in software reliability engineering: how to estimate the number of software defects; how to model software operational profiles; and how to estimate software failure rate and quantify software correctness when testing reveals no failures. Researchers will find that this book presents a reasonable summary of related topics and contains a modest amount of original and new research results. Although, due to the space limitation, methods discussed in the main text are selected, a separate section, "Remarks and Problems", is created for each chapter to address ideas and references.