Dehn twists and symplectic mapping class groups

I will introduce symplectic mapping class groups and survey what we know about them in dimension four. In particular I will talk about Seidel's result on cases where squared Dehn twist can be smoothly isotopic to identity but not symplectically isotopic to identity, on certain symplectic four manifolds.

Reference: Seidel, lectures on four dimensional Dehn twists.