This article discusses the Constrained and Unconstrained Testing Environment (CUTE), a versatile environment for testing small- and large-scale nonlinear optimization algorithms. CUTE includes a collection of test problems written in the Standard Input Format (SIF) used by the LANCELOT software package, along with tools for managing and interfacing these problems with various optimization packages. The article covers the scope of the test problem database, which includes a wide range of nonlinear optimization problems from academic and real-world sources. It also describes the classification scheme used to categorize the problems and provides tools for selecting and managing these classifications. Additionally, the article details the tools available for evaluating the decoded data from SIF files, including routines for unconstrained and bound-constrained problems, as well as general constrained problems. The article concludes with instructions on how to obtain and install CUTE, and it emphasizes the importance of user feedback for improving the environment.This article discusses the Constrained and Unconstrained Testing Environment (CUTE), a versatile environment for testing small- and large-scale nonlinear optimization algorithms. CUTE includes a collection of test problems written in the Standard Input Format (SIF) used by the LANCELOT software package, along with tools for managing and interfacing these problems with various optimization packages. The article covers the scope of the test problem database, which includes a wide range of nonlinear optimization problems from academic and real-world sources. It also describes the classification scheme used to categorize the problems and provides tools for selecting and managing these classifications. Additionally, the article details the tools available for evaluating the decoded data from SIF files, including routines for unconstrained and bound-constrained problems, as well as general constrained problems. The article concludes with instructions on how to obtain and install CUTE, and it emphasizes the importance of user feedback for improving the environment.