We used thematic analysis to investigate factors affecting decision making about gastrostomy and noninvasive ventilation (NIV) by people with Amyotrophic Lateral Sclerosis (ALS) from the viewpoint of the health care professionals (HCPs) supporting them. We conducted 20 in-depth interviews with 19 HCPs nominated by people with ALS who had made a decision to accept or decline NIV or gastrostomy. We found the main themes influencing decision making were patient-centric, caregiver-related or related to HCPs’ own beliefs, perspectives, and actions. HCPs felt patients should be, and were, in control of decision making, although caregivers and HCPs played a role. The patient’s evaluation of quality of life, the desirability of prolonging life, and acceptance of the disease and its progression by both patient and caregiver were the most important factors identified by HCPs. HCPs should be aware of the importance of multiprofessional discussions, and the potential influences (identified above) that might require discussion with patients and caregivers.