Although digital serious games are becoming increasingly popular, it remains challenging to evaluate and compare them. Based on the "DIN SPEC 91380 Serious Games Metadata Format" and the "Serious Games Information Center," we present criteria for a quality label for effective and attractive serious games. With such a label, transparent requirements for high-quality serious games should create trust and thus increase the acceptance of serious games among all stakeholders - from users to developers. A digital game is considered to be a serious game when it fulfills the following requirements. Serious games aim to accomplish goals or challenges (mental or/and physical tasks) through game actions and defined rules. The intention of serious games must be to achieve at least one additional goal, a so-called characterizing goal, e.g., improvement of player's skills or sensibilization of player's values and orientation. This characterizing goal needs to be appropriately integrated into the game. Furthermore, serious games must be interactive and should enable players to explore or influence the virtual environment (the game world). Moreover, they need to be entertaining and must elicit and maintain the player experience. In this paper, we aim to identify quality criteria for serious games and, in particular, educational games. These quality criteria can also be applied in other fields, such as healthcare, training, and simulation. We propose requirements for the three quality areas: (1) the serious part including the characterizing goal, (2) the game part maintaining the player experience, and (3) the fit between these two areas including technical implementation of the game. The first part of the paper provides background information and the motivation for this work. In the main part of the paper, the concept and criteria for a quality label are introduced. Hereby, the idea and aspects of the three dimensions are detailed. Afterward, the elaborated concept for a quality label is validated in the context of an existing educational game, Meister Cody - Talasia (Meister Cody GmbH). Finally, the results of this work are summarized and further research, development, and transfer activities are outlined.