Purpose: In intensive care units (ICUs), decisions about the continuation or discontinuation of life-sustaining treatment (LST) are made on a daily basis. Professional guidelines recommend an open exchange of standpoints and underlying arguments between doctors and families to arrive at the most appropriate decision. Yet, it is still largely unknown how doctors and families argue in real-life conversations. This study aimed to (1) identify which arguments doctors and families use in support of standpoints to continue or discontinue LST, (2) investigate how doctors and families structure their arguments, and (3) explore how their argumentative practices unfold during conversations. Method: A qualitative inductive thematic analysis of 101 audio-recorded conversations between doctors and families. Results: Seventy-one doctors and the families of 36 patients from the neonatal, pediatric, and adult ICU (respectively, N-ICU, P-ICU, and A-ICU) of a large university-based hospital participated. In almost all conversations, doctors were the first to argue and families followed, thereby either countering the doctor’s line of argumentation or substantiating it. Arguments put forward by doctors and families fell under one of ten main types. The types of arguments presented by families largely overlapped with those presented by doctors. A real exchange of arguments occurred in a minority of conversations and was generally quite brief in the sense that not all possible arguments were presented and then discussed together. Conclusion: This study offers a detailed insight in the argumentation practices of doctors and families, which can help doctors to have a sharper eye for the arguments put forward by doctors and families and to offer room for true deliberation.