A fully declarative build system is described for software build optimization. The build system supports complete control over build and test inputs and build and test operations. The build system takes inputs from a source control system. The inputs can include source code files, test files and various site files used to generate human-oriented materials for the modules. A dependencies file specifies a set of dependencies among the modules of the system. When invoked, the build system determines that changes have been made to the inputs. The system then parses the dependencies file, and based on it, determines exactly which inputs will be affected by the changes. The build system can then rebuild only those modules that will be affected by the changes, as determined from the dependencies file. Alternatively, the system may only perform testing or document generating for files which are dependent on the modified file.