Context: The merits of informal learning have been widely reported and embraced by medical educators. However, research has yet to describe in detail the extent to which informal intraprofessional or informal interprofessional education is part of graduate medical education (GME), and the nature of those informal education experiences. This study seeks to describe: (i) who delivers informal education to residents; (ii) how often they do so; (iii) the content they share; and (iv) the teaching techniques they use. Methods: This study describes instances of informal learning in GME captured through non-participant observations in two contexts: a palliative care hospice and a paediatric hospital. Analysis of 60 hours of observation data involved a process of collaborative team consensus to: (i) identify instances of informal intraprofessional and informal interprofessional education, and (ii) categorise these instances by CanMEDS Role and teaching technique. Results: Findings indicate that 84.8% of GME-level informal education that takes place in these two settings is physician-led and 15.2% is nurse-led. Organised by CanMEDS Role, findings reveal that, although all Roles are addressed by both physicians and nurses, those most commonly addressed are Medical Expert (physicians: 35.7%; nurses: 27.5%) and Communicator (physicians: 22.3%; nurses: 25.0%). Organised by teaching technique, findings reveal that physicians and nurses favour similar techniques. Conclusions: Although it is not surprising that informal interprofessional education plays a lesser role than informal intraprofessional education in GME, these findings suggest that the role of informal interprofessional education is worthy of support. Echoing the calls of others, we posit that medical education should recognise and capitalise on the contributions of informal learning, whether it occurs intra- or interprofessionally.