Behavior Trees have been used as a modelling technique to successfully develop a number of complex software systems. However, the only available formal characterisation of this technique makes heavy use of CSP, which is not very accessible for the object-oriented community where this technique could find valuable usage. This also makes Behavior Trees hard to learn, communicate, use and implement in software tools. This paper offers a comprehensive metamodel that formally describes the main areas of the Behavior Trees technique, thus improving its usability and overall value for the object-oriented community.