What Kustomize is: the overlay model

Template-free customization.

Kustomize is a template-free way to customize Kubernetes manifests. Where Helm parameterizes YAML with a templating language, Kustomize takes the opposite approach: your manifests stay plain, valid Kubernetes YAML that kubectl could apply as-is, and you layer declarative customizations — patches and transformers — on top. There are no {{ }} placeholders, no template engine, and no values.yaml; a kustomization.yaml describes how to assemble and modify a set of real manifests.

The signature pattern is bases and overlays. A base holds the shared, environment-agnostic manifests. An overlay for each environment (dev, staging, prod) references the base and applies only what differs — more replicas in prod, a different image tag, an extra label. Because everything is plain YAML, you can read any file on its own and diffs are meaningful. Kustomize is built into kubectl (kubectl apply -k), so for basic use there is nothing to install.

Why template-free

The appeal is that manifests never stop being manifests. There is no mental context-switch between YAML and a template language, no whitespace-in-Go-templates class of bugs, and tooling that understands Kubernetes YAML (schema validators, IDE completion) works on your files directly. The trade-off is less raw power than Helm for complex, distributable packages — which is exactly why many teams use Kustomize for their own env overlays and Helm for packaged third-party apps.