Modernizing legacy web applications has become an increasingly important challenge for organizations seeking to remain competitive in rapidly evolving technological environments. Legacy systems often suffer from technical debt, architectural complexity, limited scalability, poor maintainability, and dependence on outdated technologies. These characteristics make legacy applications difficult to evolve, integrate, and maintain while increasing operational costs and development effort.

Selecting an appropriate modernization strategy is a complex decision-making process involving trade-offs between software quality attributes, project risks, implementation complexity, organizational readiness, and available resources. Although numerous modernization  approaches have been proposed in both academia and industry, existing research frequently focuses on individual strategies in isolation, making it difficult for practitioners to compare alternatives in a structured manner.

This study conducts a Systematic Literature Review (SLR) to investigate modernization strategies for legacy web applications. The objective is to identify common technical and architectural challenges, examine modernization strategies reported in the literature, evaluate their impact on software quality attributes, and analyze the risks associated with modernization initiatives. The review synthesizes findings from peer-reviewed studies published between 2000 and 2024 collected from IEEE Xplore, ACM Digital Library, Google Scholar, and BTH Summon.

The results indicate that legacy systems are primarily characterized by high coupling, monolithic architectures, accumulated technical debt, outdated technologies, poor documentation, and limited scalability. Five major modernization strategies were consistently identified in the literature: incremental modernization, architectural refactoring, replatforming, rewriting, and migration to microservices architectures.

The analysis demonstrates that each modernization strategy involves significant trade-offs. Incremental modernization minimizes migration risk but often requires longer implementation periods. Refactoring improves maintainability and reduces technical debt while requiring strong system understanding. Replatforming improves infrastructure flexibility and scalability but introduces migration challenges. Rewriting provides substantial long-term benefits but is associated with high implementation risk and cost. Microservices migration offers significant scalability and deployment flexibility but introduces operational complexity related to distributed systems management, monitoring, service communication, and organizational readiness.

Based on these findings, this study proposes a structured decision support model that integrates system characteristics, quality requirements, organizational constraints, and project risks. The model provides practical guidance for consultants and practitioners when evaluating modernization alternatives in different industrial contexts and supports evidence-based modernization decisions.

The contribution of this thesis lies in consolidating fragmented modernization research and translating the findings into a structured and practical framework for modernization strategy selection.