The choice of programming language for the backend of a web application depends on various factors such as the project requirements, developer expertise, and the specific use case. Several programming languages are commonly used for backend development. Some of the popular ones include:

Java: Known for its portability and scalability, Java is widely used in enterprise-level applications.

Python: Loved for its readability and versatility, Python is commonly used in web development with frameworks like Django and Flask.

JavaScript (Node.js): JavaScript, traditionally a front-end language, is also used on the server side with the Node.js runtime, making it possible to use a single language (JavaScript) for both frontend and backend development.

Ruby: Often associated with the Ruby on Rails framework, Ruby is known for its developer-friendly syntax and rapid development capabilities.

PHP: A server-side scripting language that is often used for web development, especially with popular frameworks like Laravel.

C#: Commonly used in conjunction with the ASP.NET framework for building scalable and robust web applications, particularly in the Microsoft ecosystem.

Go (Golang): Developed by Google, Go is known for its efficiency and is used in backend development, especially in applications where performance is critical.

Scala: A language that runs on the Java Virtual Machine (JVM) and is often used in combination with the Play Framework for building scalable and reactive web applications.

Rust: Known for its focus on performance and safety, Rust is gaining popularity in backend development, particularly in systems programming.

The choice of a backend programming language often depends on factors like project requirements, developer expertise, and the specific goals of the application. Many modern web applications use a combination of backend technologies, such as microservices architecture, where different services can be written in different languages based on their specific requirements.