This article reports on issues arising from the external evaluation of a teacher development project--the Active Learning in Primary Science (ALPS) project--undertaken in eight Bristol (UK) primary schools during 2007. The ALPS project aimed to raise pupil attainment in science by developing participating teachers' skills, knowledge and enthusiasm through a programme of centre-based continuing professional development, classroom workshops, in-school consultancy and visits to a hands-on science centre. The evaluation used a combination of documentary evidence, pupil performance data, teacher interviews and observations to gain a view of the strengths, weaknesses and impact of the programme. This raised questions concerning the timing of such evaluation studies, the tensions between the agendas of the various stakeholders and the validity of any short-term apparent gains in pupil performance or teacher attitude. The article problematises the current UK government emphasis on "measuring impact" from continuing professional development activity.