*   [](/v0.1/)
*   [Get started](/v0.1/docs/get_started/)
*   Introduction

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Introduction
============

**LangChain** is a framework for developing applications powered by language models. It enables applications that:

*   **Are context-aware**: connect a language model to sources of context (prompt instructions, few shot examples, content to ground its response in, etc.)
*   **Reason**: rely on a language model to reason (about how to answer based on provided context, what actions to take, etc.)

This framework consists of several parts.

*   **LangChain Libraries**: The Python and JavaScript libraries. Contains interfaces and integrations for a myriad of components, a basic run time for combining these components into chains and agents, and off-the-shelf implementations of chains and agents.
*   **[LangChain Templates](https://python.langchain.com/docs/templates)**: A collection of easily deployable reference architectures for a wide variety of tasks. (_Python only_)
*   **[LangServe](https://python.langchain.com/docs/langserve)**: A library for deploying LangChain chains as a REST API. (_Python only_)
*   **[LangSmith](https://smith.langchain.com/)**: A developer platform that lets you debug, test, evaluate, and monitor chains built on any LLM framework and seamlessly integrates with LangChain.

![LangChain Diagram](/v0.1/assets/images/langchain_stack_feb_2024-101939844004a99c1b676723fc0ee5e9.webp)

Together, these products simplify the entire application lifecycle:

*   **Develop**: Write your applications in LangChain/LangChain.js. Hit the ground running using Templates for reference.
*   **Productionize**: Use LangSmith to inspect, test and monitor your chains, so that you can constantly improve and deploy with confidence.
*   **Deploy**: Turn any chain into an API with LangServe.

LangChain Libraries[​](#langchain-libraries "Direct link to LangChain Libraries")
---------------------------------------------------------------------------------

The main value props of the LangChain packages are:

1.  **Components**: composable tools and integrations for working with language models. Components are modular and easy-to-use, whether you are using the rest of the LangChain framework or not
2.  **Off-the-shelf chains**: built-in assemblages of components for accomplishing higher-level tasks

Off-the-shelf chains make it easy to get started. Components make it easy to customize existing chains and build new ones.

Get started[​](#get-started "Direct link to Get started")
---------------------------------------------------------

[Here's](/v0.1/docs/get_started/installation/) how to install LangChain, set up your environment, and start building.

We recommend following our [Quickstart](/v0.1/docs/get_started/quickstart/) guide to familiarize yourself with the framework by building your first LangChain application.

Read up on our [Security](/v0.1/docs/security/) best practices to make sure you're developing safely with LangChain.

note

These docs focus on the JS/TS LangChain library. [Head here](https://python.langchain.com) for docs on the Python LangChain library.

LangChain Expression Language (LCEL)[​](#langchain-expression-language-lcel "Direct link to LangChain Expression Language (LCEL)")
----------------------------------------------------------------------------------------------------------------------------------

LCEL is a declarative way to compose chains. LCEL was designed from day 1 to support putting prototypes in production, with no code changes, from the simplest “prompt + LLM” chain to the most complex chains.

*   **[Overview](/v0.1/docs/expression_language/)**: LCEL and its benefits
*   **[Interface](/v0.1/docs/expression_language/interface/)**: The standard interface for LCEL objects
*   **[How-to](/v0.1/docs/expression_language/how_to/routing/)**: Key features of LCEL
*   **[Cookbook](/v0.1/docs/expression_language/cookbook/)**: Example code for accomplishing common tasks

Modules[​](#modules "Direct link to Modules")
---------------------------------------------

LangChain provides standard, extendable interfaces and integrations for the following modules:

#### [Model I/O](/v0.1/docs/modules/model_io/)[​](#model-io "Direct link to model-io")

Interface with language models

#### [Retrieval](/v0.1/docs/modules/data_connection/)[​](#retrieval "Direct link to retrieval")

Interface with application-specific data

#### [Agents](/v0.1/docs/modules/agents/)[​](#agents "Direct link to agents")

Let models choose which tools to use given high-level directives

Examples, ecosystem, and resources[​](#examples-ecosystem-and-resources "Direct link to Examples, ecosystem, and resources")
----------------------------------------------------------------------------------------------------------------------------

### [Use cases](/v0.1/docs/use_cases/)[​](#use-cases "Direct link to use-cases")

Walkthroughs and techniques for common end-to-end use cases, like:

*   [Document question answering](/v0.1/docs/use_cases/question_answering/)
*   [RAG](/v0.1/docs/use_cases/question_answering/)
*   [Agents](/v0.1/docs/use_cases/autonomous_agents/)
*   and much more...

### [Integrations](/v0.1/docs/integrations/platforms/)[​](#integrations "Direct link to integrations")

LangChain is part of a rich ecosystem of tools that integrate with our framework and build on top of it. Check out our growing list of [integrations](/v0.1/docs/integrations/platforms/).

### [API reference](https://api.js.langchain.com)[​](#api-reference "Direct link to api-reference")

Head to the reference section for full documentation of all classes and methods in the LangChain and LangChain Experimental packages.

### [Developer's guide](/v0.1/docs/contributing/)[​](#developers-guide "Direct link to developers-guide")

Check out the developer's guide for guidelines on contributing and help getting your dev environment set up.

### [Community](/v0.1/docs/community/)[​](#community "Direct link to community")

Head to the [Community navigator](/v0.1/docs/community/) to find places to ask questions, share feedback, meet other developers, and dream about the future of LLM's.

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*   [LangChain Libraries](#langchain-libraries)
*   [Get started](#get-started)
*   [LangChain Expression Language (LCEL)](#langchain-expression-language-lcel)
*   [Modules](#modules)
*   [Examples, ecosystem, and resources](#examples-ecosystem-and-resources)
    *   [Use cases](#use-cases)
    *   [Integrations](#integrations)
    *   [API reference](#api-reference)
    *   [Developer's guide](#developers-guide)
    *   [Community](#community)