"Arrange, Act, Assert" is a testing pattern to describe the natural phases of most software tests.

• Arrange describes whatever setup is needed
• Act describes the subject's behavior that's under test (and typically only describes a single line needed to invoke that behavior)
• Assert describes the verification that the subject's behavior had the desired effect by evaluating its return value or measuring a side-effect (with a spy or mock)

Many people have identified that "Given", "When", and "Then" are more natural English descriptions of the three phrases, and as a result many BDD tools named their testing keywords accordingly.

What's most important about the concept is that the author of the test is aware that:

• Clearly separating the three phases increases the readability of the test
• Executing the three phrases in lexical order makes the test easier to understand

Separating the phrases

When mindful of the three phases, it's trivially easy to separate. Some testing frameworks support this separation, and some don't.

Most xUnit tools don't have any built-in awareness of the pattern, so it's common to see xUnit tests separate the phases with significant whitespace such as:

public void testSomething() {
  when(helloWorld.say()).thenReturn("Something Cool");

  String result = subject.say();

  assertThat(result, is("Something Cool"));
}

BDD tools like RSpec support the separation a little more formally through support functions like before and let:

describe "#say"
  before { when(helloWorld).say { "Something Cool" } } # <- Arrange
  let(:result) { subject.say } # <- Act
  it "proxies whatever HelloWorld says" do
    expect(result).to eq("Something Cool") # <- Assert
  end 
end

Some tools, like RSpec-given, jasmine-given, and mocha-gwt were designed around emphasizing the three phases:

describe "#say"
  Given { when(helloWorld).say { "Something Cool" } }
  When(:result) { subject.say }
  Then { result == "Something Cool" }
end

Maintaining lexical order

Typically it's natural to write tests in arrange-act-assert order, but some testing utilities make this unhelpfully difficult, in particular mock objects, which require their expectations be set (an Assert phase action) prior to invocation of the subject (the Act phase). Because tests with mock assertions can be awkward to read and understand, spies are often favored as an alternative.