This work represents the first full-scale attempt to provide a restrictive theory of parameters--the nature and limits of syntactic variation. Focusing on syntactic saturation, Webelhuth hypothesizes that in natural language these phenomena are subject to the "Saturation Condition." He explains the principles behind this condition and demonstrates how it imposes strong constraints on what counts as a possible parameter in natural language. Webelhuth goes on to test this theory against empirical evidence from seven modern Germanic languages: German, Dutch, English, Swedish, Norwegian, Danish, and Icelandic.