Background: The Working Alliance Inventory (WAI) and its short forms are widely used, although the properties of the therapists? versions have been little studied. Method: We examined the psychometric properties of two short forms (WAI-S-T, WAI-SR-T), and explored the creation of a psychometrically stronger short form using contemporary measure development techniques. Well-fitting items from the full 36-item WAI were identified in a development sample (131 therapists, 688 patients) using multi-level Bayesian Structural Equation Modeling, accounting for therapist rated effects. Multi-level Item Response Theory (IRT) methods aided creation of a revised short form (WAI-S-T-IRT). Factor structures of the three forms were assessed using multi-level ML estimation with robust standard errors. Results: Collinearity problems for the Goal and Task dimensions led to testing a two-factor model (Goal?Task, Bond). All three measures showed satisfactory fit; the WAI-S-T-IRT fit slightly better but differences were minor. Testing the structures in an independent sample (N?=?1117) yielded essentially the same results. No version showed strong measurement invariance. Discussion: Continued use of current therapist forms is supported; differentiation of theoretical dimensions is difficult with current measures, and may not be possible with self-report forms.