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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →The software development life cycle (SDLC) is an organizing process for taking software from an initial idea through requirements, design, implementation, testing, deployment, operation, and eventual retirement. It is not one mandatory checklist: teams group and repeat activities differently, and security should be built into the chosen approach rather than left until release.
“SDLC” can also refer to the broader information-system lifecycle, which includes the systems and operational context around software. The phase count depends on that scope and on the model a team uses.
What is the SDLC?
The SDLC is a conceptual framework for organizing the work involved in creating and supporting software. It helps teams connect decisions made early—such as what users need and what constraints apply—to later work such as testing, deployment, maintenance, and retirement.
There is no single authoritative phase list. NIST’s 2009 software-oriented overview presents seven activity groupings: Software Concept; Analysis; Design; Coding and Debugging; System Integration and Testing; Implementation; and Maintenance and Support. A separate NIST information-system security guide, published in 2004, groups the lifecycle into five broader phases: initiation; acquisition/development; implementation; operations/maintenance; and disposition. These are different descriptions for different scopes, not competing universal standards. NIST says organizations may tailor a lifecycle to their needs. (NIST, “The System Development Life Cycle (SDLC),” 2009; NIST SP 800-64 Revision 1, 2004.)
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For an introductory view, it is useful to group the work into planning and requirements, design, implementation, testing, deployment, operations and maintenance, and retirement. Treat these as connected areas of work, not gates that every project must pass through only once.
What are the main SDLC activities?
1. Planning and requirements
The team clarifies the problem to solve, intended users, constraints, and expected outcomes. Requirements may include both functional needs—what the software must do—and non-functional needs such as performance, reliability, or security. In iterative work, teams revisit assumptions and refine requirements as they learn.
2. Design
Design turns requirements into an approach for the system: its structure, components, interfaces, data handling, and other technical decisions. Security considerations belong here too; architectural choices can affect what risks exist and how they can be addressed.
3. Implementation
Developers build or configure the software and its components. Coding is only one part of implementation: the team also needs practices for protecting source code and dependencies and for producing software in a controlled way.
4. Testing
Testing checks whether the software meets its requirements and whether its parts work together. The NCCoE’s notional DevSecOps reference model describes automated suites that can include unit, integration, regression, smoke, and user-acceptance tests. It also includes assessment of functional and non-functional requirements, security, and integration concerns. The test mix should fit the product and its risks; the NCCoE model is an example, not a universal SDLC taxonomy. (NIST NCCoE, “Notional Reference Model for DevSecOps for Demonstration of NIST SSDF Updated”.)
5. Deployment and release
Release and deployment make software available in its intended environment. Teams need to account for how a release is prepared, delivered, and operated; the specifics depend on the product and deployment context. In an iterative lifecycle, deployment is not necessarily the final activity because operational feedback may lead to further planning and changes.
6. Operations and maintenance
After deployment, the software must be operated and maintained. Teams respond to issues, make changes, and consider how operational experience should shape future work. The lifecycle therefore continues after the first release.
7. Retirement or disposition
Eventually, software or its supporting system may be withdrawn or replaced. A lifecycle that includes disposition makes room for that end state rather than treating ongoing operation as guaranteed indefinitely. NIST’s broader information-system lifecycle guide explicitly includes disposition.
How do SDLC models differ?
A lifecycle describes the work a team needs to organize; a model describes how that work is structured and revisited. NIST’s software-development overview names Waterfall, Spiral, and Evolutionary Prototyping as examples of possible models. The sources do not establish a universal winner or support a general ranking by speed, cost, risk, or ability to handle change. A team should choose and tailor an approach to its project rather than assume one model fits every situation. (NIST, “The System Development Life Cycle (SDLC),” 2009.)
- Waterfall: one of the models named by NIST; the cited overview does not establish that it is best or worst for a particular project.
- Spiral: another model named by NIST; the cited overview does not provide a comparative performance claim.
- Evolutionary Prototyping: a third model named by NIST; the cited overview does not establish a universal advantage over the other examples.
Rather than treating these names as a scorecard, ask how the proposed approach will handle the project’s requirements, feedback, testing, releases, operations, and security responsibilities. A tailored lifecycle can preserve the necessary work while changing how activities are grouped or repeated.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where does security fit in the SDLC?
Security belongs throughout software development, not only in a final test or review. NIST’s Secure Software Development Framework (SSDF) Version 1.1, published February 3, 2022, says few SDLC models explicitly address software security in detail and recommends integrating secure-development practices into each SDLC implementation. NIST says this should help reduce vulnerabilities in released software, mitigate the impact of potential exploitation, and address vulnerability root causes; it does not guarantee that software will be vulnerability-free. (NIST SP 800-218, “Secure Software Development Framework (SSDF) Version 1.1”.)
SSDF groups its practices into four areas. These are practice groups, not additional lifecycle phases:
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- Prepare the Organization (PO): establish organizational readiness for secure development.
- Protect the Software (PS): protect software and the components used to build it.
- Produce Well-Secured Software (PW): apply practices that support secure software production.
- Respond to Vulnerabilities (RV): address vulnerabilities that remain or are discovered.
This framing helps distinguish two related ideas: the SDLC organizes the work, while SSDF practices add security considerations across that work. Teams can apply the practices within different lifecycle models.
What does an iterative DevSecOps lifecycle look like?
The NCCoE’s notional DevSecOps reference model uses seven named phases: Plan, Develop, Build, Test, Release, Deploy, and Operate. It is explicitly iterative: feedback from later activities can inform reevaluation and planning, rather than the team treating a release as the end of the process. This is one reference model, not a fixed sequence that defines every SDLC. (NIST NCCoE, “Notional Reference Model for DevSecOps for Demonstration of NIST SSDF Updated”.)
The model is useful for seeing how development, testing, release, deployment, and operation can connect with feedback. It also shows why security is not a separate final phase: security and integration concerns are part of the testing activities, while secure-development practices apply across the lifecycle.
How should a team use an SDLC?
Use the lifecycle as a way to make work and responsibility visible, not as paperwork for its own sake. A practical starting point is to identify the work the product requires, decide how the team will organize and revisit it, and define how testing, operations feedback, and secure-development practices will be included. The appropriate structure depends on the project; the existence of phase names alone does not establish that the work is complete or effective.
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