To my knowledge, the only keys to the nymphal stages of the European species of Pentatomoidea (shield bugs) appear in Shchytnyky by V. G. Puchkov (1961), volume 21 of Fauna of Ukraine. Although the species included were only intended to encompass the fauna of Ukraine, these keys can still be useful for the rest of Europe. With this translation, I hope to make these keys available to a wider European audience.
Lars Willighagen 
Puchkov, V. G. (1961). Shchytnyky. In Fauna of Ukraine (Vol. 21, Issue 1). Naukova dumka. http://www.izan.kiev.ua/fau-ukr/vol21iss1.htm
Пучков, В. Г. (1961). Щитники. In Фауна України (Vol. 21, Issue 1). Наукова думка. http://www.izan.kiev.ua/fau-ukr/vol21iss1.htm