On iterated forcing at successors of regular cardinals

We investigate the problem of when $\leq\lambda$--support iterations of $<\lambda$--complete notions of forcing preserve $\lambda^+$. We isolate a property -- {\em properness over diamonds} -- that implies $\lambda^+$ is preserved and show that this property is preserved by $\lambda$--support iterations. We close with an application of our technology by presenting a consistency result on uniformizing colorings of ladder systems on $\{\delta<\lambda^+:\cf(\delta)=\lambda\}$ that complements a theorem of Shelah.