During the development of any biological structure, cells communicate with each other and, at each stage, existing cells guide the maturation of new additions. For every developing cell an extracellular source of information is vital to modulate the cell’s own intrinsic processes. One of the main aims of my own research over the past 30 years has been to understand how cell-cell interactions regulate development of the mammalian brain. In this presentation I will describe some examples of this research, particularly those inspired by Colin Blakemore and his group. Over the last 25 years our work has focussed mainly on the development of the embryonic brain, using the mouse as the model species. One of our main questions has been: how do axons from the developing thalamus navigate their complex route through the subpallium to reach appropriate target cells in the cerebral cortex? It is clear that thalamic axons do not possess detailed intrinsic information that is sufficient for them to navigate along the tortuous path to the cortex. Instead, they rely on cues from the environment that they growth into, but what are those cues? A guidewire is a thin flexible wire inserted into a tortuous space to act as a guide for insertion of subsequent bulkier material. By analogy, in the developing brain it has long been hypothesized by researchers including Colin Blakemore, Zoltan Molnar, Christine Metin, Dennis O’Leary and others that the mechanisms by which thalamic axons attain the cortex include guidance by previously-generated guide-axons, for example reciprocal projections from intermediate structures, or from the cortex itself, back towards the thalamus. Evidence supporting this idea has come largely from observations in vivo and from in vitro experiments. Here I shall present in vivo evidence from conditional mouse mutants providing further support for the importance of the descending corticothalamic axons in enabling ascending thalamic axons to cross the pallial-subpallial boundary into the cortex. In addition, I shall show that projections from prethalamus to thalamus are important for the fine organisation of thalamic axons emerging from the thalamus. The findings emphasize the importance of interactions between reciprocal axon projections in the development of brain connections. Another source of guidance for thalamic axons as they steer towards the cortex might be diffusible molecules that attract them towards the cortex and/ or repel them from inappropriate areas. In fact, our work has shown remarkably little evidence that diffusible molecules from the cortex attract thalamic axons, although the cortex does release chemotrophic molecules that stimulate the overall growth of thalamic axons. There is better evidence for molecules that repel thalamic axons from inappropriate regions as they cross towards the cortex. Overall, the results strengthen the notion that thalamocortical pathfinding involves numerous guidance cues operating at a series of steps along the route. They highlight the critical importance of cell-cell interactions in wiring the embryonic brain.