This paper introduces a unified approach to robustness analysis of nonlinear systems, time-varying systems, and systems with uncertain parameters. The approach is based on integral quadratic constraints (IQC's), which are used to describe the behavior of system components. A stability theorem for systems described by IQC's is presented, which simplifies the use of multipliers and the treatment of causality. The paper also discusses a systematic computational approach and compares it with other methods of stability analysis. Finally, a list of IQC's for important types of system components is provided. The main contributions include the development of a general framework for stability analysis, the introduction of soft IQC's, and the demonstration of the equivalence between quadratic stability and IQC-based stability analysis in certain cases.This paper introduces a unified approach to robustness analysis of nonlinear systems, time-varying systems, and systems with uncertain parameters. The approach is based on integral quadratic constraints (IQC's), which are used to describe the behavior of system components. A stability theorem for systems described by IQC's is presented, which simplifies the use of multipliers and the treatment of causality. The paper also discusses a systematic computational approach and compares it with other methods of stability analysis. Finally, a list of IQC's for important types of system components is provided. The main contributions include the development of a general framework for stability analysis, the introduction of soft IQC's, and the demonstration of the equivalence between quadratic stability and IQC-based stability analysis in certain cases.