Implementation is the part of the process where software engineers actually program the code for the project.

Implementation is the realization of an application, or execution of a plan, idea, model, design, specification, standard, algorithm, or policy.

In computer science, an implementation is a realization of a technical specification or algorithm as a program, software component, or other computer system. Many implementations may exist for a given specification or standard. For example, web browsers contain implementations of World Wide Web Consortium-recommended specifications, and software development tools contain implementations of programming languages.

In the IT Industry, implementation refers to post-sales process of guiding a client from purchase to use of the software or hardware that was purchased. This includes Requirements Analysis, Scope Analysis, Customizations, Systems Integrations, User Policies, User Training and Delivery. These steps are often overseen by a Project Manager using Project Management Methodologies set forth in the Project Management Body of Knowledge. Software Implementations involve several professionals that are relatively new to the knowledge based economy such as Business Analysts, Technical Analysts, Solutions Architect, and Project Managers.

In political science, implementation refers to the carrying out of public policy. Legislatures pass laws that are then carried out by public servants working in bureaucratic agencies. This process consists of rule-making, rule-administration and rule-adjudication. Factors impacting implementation include the legislative intent, the administrative capacity of the implementing bureaucracy, interest group activity and opposition, and presidential or executive support.

A programming language implementation is a system for executing programs written in a programming language.

There are two general approaches to programming language implementation:

Notice that a compiler does not directly execute the program. Ultimately, in order to execute a program via compilation, it must be translated into a form that can serve as input to an interpreter.

When a piece of computer hardware can interpret a programming language directly, that language is called machine code. A so-called native code compiler is one that compiles a program into machine code. Actual compilation is often separated into multiple passes, like code generation (often in for of assembler language), assembling (generating native code), linking, loading and execution.

If a compiler of a given high level language produces another high level language it is called translator (source to source translation), which is often useful to add extensions to existing languages or to exploit good and portable implementation of other language (for example C), simplifying development.

Many combinations of interpretation and compilation are possible, and many modern programming language implementations include elements of both. For example, the Smalltalk programming language is conventionally implemented by compilation into bytecode, which is then either interpreted or compiled by a virtual machine (most popular ways is to use JIT or AOT compiler compilation). This implementation strategy has been copied by many languages since Smalltalk pioneered it in the 1970s and 1980s.