If automated vehicles are to be introduced on public roads, they need to be able to communicate appropriately with other road users. This can be done using various interfaces and by communicating various messages. Previous research has mainly investigated design of such communication for automated passenger cars. It is, however, currently largely unknown how corresponding communi- cation should be designed for heavy automated vehicles. Scania and RISE are collaborating in a research project on what signals need to be displayed for heavy automated vehicles when they get introduced to public roads. This thesis focuses on design of an external human-machine interface (eHMI) that conveys that a truck is operated in automated driving mode. It explored various types of message contents (abstract lights, text, symbols) as well as the effect of placement of eHMI (grille, under windshield, above windshield) and distance on understanding of the message. The emphasis was on the communi- cation to pedestrians in a crossing scenario. The thesis work was split into three design iterations according to the ”De- sign Thinking” methodology. The first iteration investigated the most preferred content types. The second investigated the effect that the screen placement on the truck had on the comprehensibility of the sign with regards to distance. The third and last iteration meant creating physical prototypes of low fidelity corresponding to the concepts from the second iteration, installing them on a truck and then evaluating them on a test track. The final evaluation was of an exploitative character and involved experts in the field of HMI design. The results showed that it was hard to interpret signals based on colors and abstract lights only. Symbols were also hard to interpret and should only be used when there is a standard for symbols for automated vehicles. Using text, on the other hand, made the message more clear and was easier to understand independently of the distance. As for the placement of the eHMI, the results show that there are preferences to using the middle and upper part of the truck.