Visual modeling languages are appealing because "a picture tells more than a thousand words". The UML is such a visual language and is well known for its graphical notation and diagramming techniques. However, pictures often only tell half of the story and additional information needs to be captured in a different way. For this reason the UML includes the Object Constraint Language (OCL), a textual language which allows a UML modeler to specify these additional constraints.This tutorial shows the importance of constraints as an object-oriented modeling technique and how they add value to the visual modeling techniques and help "telling the complete story". The OCL language itself and the connection with the visual UML diagrams is thoroughly explained. The final part of the tutorial will show how one can achieve a good balance between the visual and constraint modeling in UML.