Cannabis and tobacco are frequently co-used (e.g., smoked in a joint), yet most research on cannabis use neglects the role of tobacco, resulting in an incomplete understanding of its health effects. This gap is particularly problematic in European countries like the Netherlands, where cannabis is typically consumed with tobacco and can cause serious problems when comparing results from Europe with studies from for example the USA, where co-use is less common. Furthermore, co-users often underestimate their tobacco intake, are more vulnerable to relapse when quitting, experience more tobacco-related health problems, and experience worse clinical outcomes, yet they remain largely invisible in tobacco control strategies. To understand cannabis-tobacco co-use, accurate measurement is key. Hence, this research project – which is a collaboration between researchers from Erasmus University Rotterdam and the University of Bath (UK) – aims to develop a measurement tool for cannabis-tobacco co-use that is efficient, population-suitable, and culturally sensitive.