This book reviews recent results on low-dimensional quantumfield theories and their connection with quantum grouptheory and the theory of braided, balanced tensorcategories. It presents detailed, mathematically preciseintroductions to these subjects and then continues with newresults. Among the main results are a detailed analysis ofthe representation theory of U (sl ), for q a primitiveroot of unity, and a semi-simple quotient thereof, aclassfication of braided tensor categories generated by anobject of q-dimension less than two, and an application ofthese results to the theory of sectors in algebraic quantumfield theory. This clarifies the notion of "quantizedsymmetries" in quantum fieldtheory. The reader is expectedto be familiar with basic notions and resultsin algebra.The book is intended for research mathematicians,mathematical physicists and graduate students.