Mobile Health Units are often used to provide health care in unstable situations, such as armed conflicts, where fixed services cannot function for reasons of security. They are, however, a controversial way of providing health care, because of their cost, their irregular service provision and their logistical complexities. Drawing on the experience of the International Committee of the Red Cross (ICRC) and on the relevant literature, this Network Paper provides a decision-making framework for health care workers considering whether to use Mobile Health Units. The paper gives an overview of the place of MHUs in a health care system, and provides the theoretical background to the decision-making process around how and when to set them up. It also elaborates on the strengths and weaknesses on Mobile Health Units, and uses practical examples both from the literature as well as from the authors' own field of experiences to illustrate its argument. The paper concludes that, although a logical approach in contexts where traditional permanent (fixed) health structures are unavailable, absent, overburdened or dysfunctional, other options should be considered before embarking on the wholesale substitution of health care services through Mobile Health Units or other structures.