As the name suggests optional parameters are parameters which may or may not be passed to a method when the method is called. If they're not passed then a default value is used instead. Optional parameters must always be the last parameters in a method's parameter list i.e. they can't come before a non-optional parameter. For more detail check this link: http://msdn.microsoft.com/en-gb/library/dd264739.aspx

Parameters that you don't have to pass ;-) public void MyMethod(string strFirstname string strLastname) When calling you need to pass both parameters public void MyMethod(string strFirstname string strLastname=Sturkenboom) When calling you ony have to pass the first parameter; second parameter if omitted in call is set to a default.