Entanglement of quantum states and its measures play an important role in many areas of theoretical physics. Some techniques about how to deal with entanglement in QFT will be discussed. In particular, the strong subadditivity play the crucial role in the analysis of the "c-theorems" in 1+1 and 2+1 dimensions. We will also consider the twist fields and how they are employed to find analytic results for the entanglement entropies of disjoint intervals and the negativity (a measure of entanglement for mixed states) 1+1 CFTs.