The scope of this thesis is to introduce a transport theory in the weak tunnelling limit where we include also the cotunnelling transport regime. The main advance of this thesis is that we rigorously take into account the coherences up to this order. We achieved this through solving the related cotunnelling energy integrals. With the help of a generalized master equation, we can describe interference effects accurately in systems like double quantum dots. A special emphasis is on so-called spin resonances without spin splitting.