This chapter provides a complete developer-level guide on designing and analyzing cross-chain smart contract systems using Ethereum-compatible technologies. Before delving into basic blockchain interoperability concepts like state dependencies, execution boundaries, and smart contracts as message relayers, it provides a full setup guide for Node.js, Truffle, and Ganache. This chapter applies theory to practice by introducing developers to fundamental Solidity syntax and reusable design ideas. The tutorial, which focuses on lock-mint and Oracle-based flows, retry logic, and JSON-RPC-based simulations, contains extensive implementation approaches for interoperable smart contracts. Two industry-leading interoperability protocols, Chainlink CCIP and LayerZero ULN, are simulated step-by-step together with contract code, deployment scripts, and interaction testing on simulated EVM chains. To illustrate sender–receiver logic, migration scripts, and security design principles, a token bridge prototype uses a lock-mint bridge. Static analysis tools, circuit breakers, verified messaging, and nonce tracking are among the security best practices that have been discussed in the chapter. This chapter has covered a number of security best practices, including nonce tracking, circuit breakers, verified communications, and static analysis tools. This chapter concludes with an overview of the tools and recommended project architectures, giving developers the confidence they need to create, test, and evolve cross-chain apps in safe environments.