This is the abstract of the paper "Operator theoretic properties of semigroups in terms of their generators " by Soenke Blunck and Lutz Weis. For the whole paper send the command get evolve-l 01-00027 to: "listserv@uni-karlsruhe.de". Instead you may get the paper at "http://ma1serv.mathematik.uni-karlsruhe.de/evolve-l/index.html" too. Abstract: Let $(T_t)$ be a $C_0$ semigroup with generator $A$ on a Banach space $X$ and let ${\cal A}$ be an operator ideal, e.g. the class of compact, Hilbert-Schmidt or trace class operators. We show that the resolvent $R(\lambda,A)$ of $A$ belongs to ${\cal A}$ if and only if the integrated semigroup $S_t:=\int_0^t T_s ds$ belongs to ${\cal A}$. For analytic semigroups, $S_t\in{\cal A}$ implies $T_t\in{\cal A}$, and in this case we give precise estimates for the growth of the ${\cal A}$-norm of $T_t$ (e.g. the trace of $T_t$) in terms of the resolvent growth and the imbedding $D(A) \hookrightarrow X$. 1991 Mathematics Subject Classification: 47A60 , 47B10 , 47D06 ------------------ http://www.uni-karlsruhe.de/~listserv/ ------------------