Kotlin Multiplatform Mobile (KMM): The Complete Developer Guide

What is Kotlin Multiplatform Mobile (KMM)?

KMM is a framework that bridges the gap between native and cross-platform app development, offering the performance of native with the efficiency of code sharing. The framework enables developers to:

• Write shared logic once in Kotlin and run it on both Android and iOS.
• Keep native UI layers for each platform, ensuring platform familiarity.
• Reduce duplication, streamline updates, and improve code maintainability.
  

Key takeaways

• Kotlin Multiplatform Mobile (KMM) lets you build Android and iOS apps from a single codebase while retaining native UI performance.
• Share business logic across platforms while keeping platform-specific UI and features.
• Ideal for projects where cost, time-to-market, and consistency matter.
• Backed by JetBrains, strong community support, and proven real-world adoption.

Structure and architecture

your-project/

├── shared/

│   ├── src/

│   │   ├── commonMain/

│   │   ├── androidMain/

│   │   └── iosMain/

├── androidApp/

└── iosApp/

Project structure

A typical KMM project consists of three main modules:

1. shared: Contains the code that is common between Android and iOS. This includes business logic, data management, and network requests.
2. androidApp: The Android-specific module where the Android UI and platform-specific implementations reside.
3. iosApp: The iOS-specific module for iOS UI and platform-specific code.
   
   

Shared code

The shared module typically includes the following:

• commonMain: Contains the actual shared code across platforms.
• androidMain: Android-specific code.
• iosMain: iOS-specific code.
  
  

Architecture

KMM supports popular architectural patterns such as 

• MVVM (Model-View-ViewModel) and 
• MVP (Model-View-Presenter). 

The shared code handles the core logic, while the platform-specific modules manage the UI.

Popular use cases for KMM

Create a Kotlin Multiplatform app with shared logic and native UI

One of the major Kotlin Multiplatform use cases is sharing code between mobile platforms. You can share application logic between iOS and Android apps and write platform-specific code only when you need to implement a native UI or work with platform APIs.

This pattern works excellently for apps that want to feel native while avoiding business logic duplication. Music streaming services often take this route - sharing core playback logic, playlist management, and recommendation systems while implementing platform-specific UI controls. This gives users a familiar interface while maintaining consistent functionality underneath.

Create a Compose Multiplatform app with shared logic and UI

With the Compose Multiplatform UI framework, you can push the code-sharing capabilities of Kotlin Multiplatform beyond application logic. You can implement the user interface once and then use it for all the platforms supported by Compose Multiplatform.

When businesses need identical experiences across platforms, KMM shines by sharing both business logic and user interface. Banking apps frequently use this approach to ensure consistent transaction flows and security features. The unified experience helps users trust the application, while developers benefit from maintaining a single codebase for core functionality.

Share a piece of logic

The application will include a module with shared code for both the iOS and Android platforms. The business logic and data access layers will be implemented only once in the shared module, while the UI of both applications will be native.

Some applications benefit from sharing only specific components. E-commerce apps commonly share cart management and payment processing logic while keeping their UI and platform-specific features (like Apple Pay or Google Pay) native. This balanced approach lets teams optimize for both code reuse and platform-specific capabilities.

At-a-glance: Popular KMM use cases

Getting started with KMM

Install the necessary tools

Check your environment

To make sure everything works as expected, install and run the KDoctor tool:

brew install kdoctor

2. After the installation is completed, call KDoctor in the console:

kdoctor

3. If KDoctor diagnoses any problems while checking your environment, review the output for issues and possible solutions:

Fix any failed checks ([x]). You can find problem descriptions and potential solutions after the * symbol.

Check the warnings ([!]) and successful messages ([v]). They may contain useful notes and tips, as well.

Why use Kotlin Multiplatform Mobile (KMM)?

Benefits of KMM

Code reusability

Write once and run on both Android and iOS, which significantly reduces development time and effort.

Maintainability

Easier to maintain and update the shared logic as there's only one codebase for both platforms.

Consistency

Ensures consistent behaviour across both platforms by using shared business logic.

Flexibility

Allows for platform-specific code and native UI, ensuring that the app still feels native to the users.

Community and support

Strong community support and extensive documentation make it easier to get started and find solutions.

Real-world adoption stories

Many companies have successfully adopted KMM:

Best practices for KMM development

• Start small — migrate one feature at a time.
• Keep UI native for complex platform-specific interactions.
• Use dependency injection for testability.
• Continuously test on both platforms.
• Integrate mobile security testing early (e.g., Appknox for compliance and privacy validation).

Final thought

Kotlin Multiplatform Mobile is a pragmatic middle ground between full native and fully cross-platform development, offering a powerful way to streamline app development for Android and iOS. 

By sharing code between platforms, developers can reduce development time, ensure consistency, and maintain flexibility for native experiences. It reduces engineering overhead without compromising on performance or user experience, making it a strong choice for both startups and enterprises.

So, whether you're starting a new project or looking to optimize your existing workflow, KMM provides a compelling solution for modern mobile development.

Frequently asked questions (FAQs)

1. Is Kotlin Multiplatform Mobile stable for production?

Yes. Kotlin Multiplatform Mobile has reached a level of maturity where it’s used in production by major brands like McDonald's, VMware, Philips, and Cash App. Its stability comes from JetBrains’ active development and a robust release cycle, ensuring compatibility with the latest Android and iOS SDKs. 

While certain UI-related features (like Compose for iOS) are still evolving, the shared logic layer is production-ready and proven at scale.

2. How much code can be shared?

Typically, 60–90% of your application’s code can be shared between Android and iOS, including 

• Business logic, 
• Data models, 
• Network calls, and 
• Utility functions. 

The remaining percentage usually involves platform-specific UI and integrations — such as push notifications, payment gateways, or OS-level APIs — that are best implemented natively.

3. How does KMM differ from Flutter or React Native?

KMM focuses on sharing core business logic while leaving the UI layer fully native for each platform, meaning your app looks and behaves exactly like other native apps. 

Flutter and React Native, on the other hand, render their own UI components, replacing the native UI layer entirely. 

This means KMM preserves platform-specific UX and performance benefits, while Flutter/React Native aim for a single cross-platform UI experience.

4. Does KMM support Compose for iOS?

Yes. KMM supports Compose for iOS through Compose Multiplatform, which lets you build a shared UI layer for Android and iOS using the same code. 

However, the iOS side is still relatively young compared to Android, so teams should evaluate performance and feature completeness before committing. It’s an excellent option for projects that need identical UI across platforms without maintaining two separate UI codebases.

5. Can I migrate an existing native app to KMM?

Yes, absolutely. 

Migration can be incremental, which means you don’t need to rebuild your entire app from scratch. 

You can start by moving isolated features or business logic — like authentication, API calls, or data handling — into the shared module, while keeping your existing native UI intact. This approach reduces risk and allows for gradual adoption.

6. What about app security with KMM?

Even though KMM centralizes your business logic, that shared code still needs thorough security testing. Vulnerabilities in the shared module can impact both Android and iOS apps simultaneously. 

Integrating tools like Appknox ensures continuous scanning for security flaws, privacy violations, and compliance risks, covering both the shared logic and platform-specific layers.