Numerous studies have shown that in vivo a key feature of microtubules is their dynamic instability — rapid switching between phases of elongation and shortening. For technical reasons, these studies have all examined microtubules at the cell periphery. Do microtubules in the cell interior behave the same way? Yulia Komarov and co-workers have addressed this question, using several novel experimental approaches (seep. 3527). These include analysis of the growth of fluorescently tagged microtubules through laser-photobleached centrosomes, direct observation of microtubules in centrosome-containing cytoplasts, tracking of GFP-tagged microtubule-binding proteins, and sequential subtraction analysis of regional differences in microtubule behaviour. The major finding is that, whereas at the periphery microtubule plus ends show dynamic instability, in the cell interior shortening events are infrequent and so the plus ends continue to grow. The authors propose that newly nucleated microtubules grow out rapidly from centrosomes towards the cell periphery and, once there, experience a `boundary effect' that induces catastrophe (the switch from elongation to shortening)followed by cycles of elongation and shortening characteristic of peripheral microtubules.