This research uses the construct of group development (GD) to distinguish highly developed workgroups and teams from mere social aggregates. The aims were to develop a scale capable of measuring this basic emergent process and to study the scale's reliability and construct validity (content, factorial, convergent and criteria). Data concerning the GD, oth- er related processes (entitativity and group identification) and team outputs (group performance and team effectiveness) were gathered from four successive studies (4099 participants belonging to 521 workgroups in 13 organizations). All the studies were carried out using a cross-sectional and correlational design. The results revealed an one-dimensional solution for the proposed measurement scale, which showed adequate reliability and validity. The scale is not only practical (quick and easy to apply) but also useful for group managers and leaders, since it provides them with a tool for determining the extent to which their groups are actually functioning as highly developed groups.