# defining equations for secant varieties

Could anyone here give me some reference on finding defining equations for secant variety of any projective varieties? Ex: secant variety of Segre, Grassmanian ...

There is an advanced monograph by F.L.ZAK on exactly this subject: "Tangents and secants of algebraic Varieties" (AMS, 1993, Translations of Mathematical Monographs 127). A free pdf version is online