Friends are important for well-being, yet who people consider to be friends is much less evident. With a novel method to examine social relationships while decreasing respondent burden (GENSI), we obtained detailed information on 25 relationships of each of the 706 respondents from a representative sample of Dutch women (age 18-40). Specifically, we examined: i) which kind of personal relations were most often identified as friends, ii) to what extent relationship strength measured by closeness and frequency of contact (face-to-face and other forms) could predict friendships, and iii) whether the importance of relationship characteristics in determining friendships differed with age. People met in school were most likely (>70%), and family members were least likely to be friends (20%). Friendships were most often close relationships with more non-face-to-face contact, whereas meeting in person was less predictive. Even with three measures of relationship strength, it was difficult to predict whether somebody was considered a friend, particularly among family. With advancing age, women reported fewer friends, but closeness and frequency of contact were similarly important for friendships at different ages. Friendships were difficult to predict, implying that individuals understand friendships in different ways, and often overlapped with other social roles like family and colleagues. In conclusion, even though friends are of considerable importance for people’s well-being, there is still much to learn about what makes a friend. These results reiterate the difficulty in getting a grip on the term “friend”, and are important for those scholars working on social relations.