Background: Despite widespread age-of-sale restrictions on tobacco, adolescents continue to obtain cigarettes and experiment with smoking. This mixed-methods study aimed to understand how European adolescents access cigarettes and how the policy context may influence this process, using a realist evaluation approach. This is the first study to assess access to cigarettes across various European contexts. Methods: A survey of 4104 students was combined with qualitative data from focus groups among 319 adolescents aged 14-19 across seven European countries. Data were synthesized to explore mechanisms via which young people obtain cigarettes despite age-of-sale restrictions. Results: While purchasing cigarettes from supermarkets was widely regarded as difficult, many participants purchased cigarettes from noncompliant retailers (often in smaller shops or cafes). Other contra-mechanisms included circumventing age checks, proxy purchases, and/or social sources. Dominant forms of access differed across the seven contexts, with direct purchases more common where perceived enforcement was low (eg, Belgium) and proxy purchases more important where perceived enforcement of age-of-sale laws was high (eg, Finland). The effectiveness of age-of-sale restrictions in reducing youth access appears to be influenced by a range of contextual factors including retailer compliance, the availability of vending machines, and the specific minimum age-of-sale. Conclusions: Our findings illustrate the relevance of programme theory in understanding the contra-mechanisms that undermine the effectiveness of age-of-sale laws in discouraging youth smoking. Young people's access to cigarettes could be further limited by addressing these contra-mechanisms, including an increase in the legal sales age (particularly in Belgium), banning vending machines, and strengthening enforcement. Implications: Despite widespread implementation of age-of-sale laws, a substantial proportion of minors continue to access cigarettes. Young people use a number of contra-mechanisms to circumvent age-of-sale restrictions. These include accessing cigarettes via social sources, proxy sales or by circumventing age checks. Our findings show that in contexts where perceived enforcement of age-of-sale restrictions is high, young people are more reliant on irregular forms of access such as proxy sales. Young people's access to cigarettes may be further reduced by policy interventions that address these contra-mechanisms - for example, banning vending machines, strengthening enforcement of age-of-sale laws, and increasing the minimum age-of-sale.