Background: Descriptive studies of cancer family caregivers demonstrate role-related psychosocial and physical burden; however, little is known about which factors contribute to or obviate burden. Systematic reviews of caregiver intervention studies demonstrate mixed results, perhaps because some caregiver needs are still unknown and not adequately addressed. The purpose of our study was to explore the lived experience of being a caregiver for an adult with lung or colon cancer, so as to guide the development of future intervention studies. Method: Using phenomenologic methods, open-ended interviews were conducted at a chemotherapy clinic, New Haven, CT with 135 caregivers to adults with lung or colon cancer. Interviews were audio-recorded and transcribed. Thematic analysis was conducted with transcripts coded, reviewed, and recoded multiple times. The final 69 codes were reduced to 13 code clusters (thematic categories) distributed among 4 themes. Results: Four inter-related themes emerged: 1) becoming a caregiver; 2) new and altered relationships; 3) personal responses to caregiving and 4) antecedents and social context. Caregivers describe hearing the cancer diagnosis as "life-changing". The cancer creates the context for the caregiver's relationships (with the patient, self, others, and the healthcare system), and cognitive, behavioral, affective, and spiritual responses. The caregiver's antecedent experiences and social support network form the foundation for their perceptions of the diagnosis, relationships, and personal responses. Conclusion: This study implicates several intervention components to be developed and tested as favorably supporting caregivers, namely, reinforcing positive aspects of caregiving, cultivating open communication, and acknowledging the prior experiences and social foundation of the caregiver's life that can be supportive or burdensome. © Copyright 2012, Mary Ann Liebert, Inc.