A functional behaviour test system for a target computer program, the target computer program including data defining user interface functionality encoded in a data repository, the test system including: a definition system arranged to receive, for the target computer program, a selection of a functional behaviour test definition, the definition system further being arranged to receive an input on one or more parameters for the selected behaviour test definition to define a functional behaviour test for the target computer program; a validation system arranged to validate the functional behaviour test in dependence on the data defining user interface functionality to constrain the functional behaviour test to possible actions that may occur within the target computer program, the possible actions being actions determined as being reachable or possible from the data defining the user interface functionality encoded in the data repository, an action comprising a user interaction with the user interface or an action concerned with the environment or input from an external system; a data repository to store the validated functional behaviour test; a test interface system to provide access by an automated execution environment to the validated functional behaviour test in the data repository for testing of the validated functional behaviour test against the target computer program, wherein the functional behaviour test comprises a user scenario and is defined with respect to a user input, the test system being arranged to translate the user scenario into one or more actions to be performed by the automated execution environment to simulate the user input and communicate said one or more actions to the automated execution environment to test the target computer program, the system being further arranged to manipulate the actions to be communicated to the automated execution environment to cause non-standard execution associated of an aspect of the target computer program associated with the validated functional behaviour test, wherein the non-standard execution comprises changing emphasis of user interface functionality when the target computer program is executed by the automated execution environment to highlight the user interface functionality associated with the validated functional behaviour test. a definition system arranged to receive, for the target computer program, a selection of a functional behaviour test definition, the definition system further being arranged to receive an input on one or more parameters for the selected behaviour test definition to define a functional behaviour test for the target computer program; a validation system arranged to validate the functional behaviour test in dependence on the data defining user interface functionality to constrain the functional behaviour test to possible actions that may occur within the target computer program, the possible actions being actions determined as being reachable or possible from the data defining the user interface functionality encoded in the data repository, an action comprising a user interaction with the user interface or an action concerned with the environment or input from an external system; a data repository to store the validated functional behaviour test; a test interface system to provide access by an automated execution environment to the validated functional behaviour test in the data repository for testing of the validated functional behaviour test against the target computer program, wherein the functional behaviour test comprises a user scenario and is defined with respect to a user input, the test system being arranged to translate the user scenario into one or more actions to be performed by the automated execution environment to simulate the user input and communicate said one or more actions to the automated execution environment to test the target computer program, the system being further arranged to manipulate the actions to be communicated to the automated execution environment to cause non-standard execution associated of an aspect of the target computer program associated with the validated functional behaviour test, wherein the non-standard execution comprises changing emphasis of user interface functionality when the target computer program is executed by the automated execution environment to highlight the user interface functionality associated with the validated functional behaviour test.