Inclusion Criteria: - Must be ≥18 years of age at the time of consent - Must be able to provide written informed consent personally or by legally authorized representative - Must have 1 of the following histologically confirmed Non-Hodgkin Lymphoma (NHL) subtypes: - Diffuse large B-cell lymphoma (DLBCL), NOS; or DLBCL high-grade lymphoma, not otherwise specified (NOS); or DLBCL high-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements with DLBCL histology (double/triple-hit lymphoma) - Epstein-Barr virus-positive or composite DLBCL are allowed - Follicular lymphoma (FL) - Must have been previously treated with ≥ 1 prior systemic therapy (e.g., chemotherapy, immunotherapy, or chemoimmunotherapy) - For first relapsed/refractory (R/R) DLBCL cohort, participant must have confirmed R/R disease within 90 days prior to study enrollment and must intend to initiate 2L systemic treatment - For first R/R FL cohort, participant must have confirmed R/R disease (grade 1 to 3B or transformed) within 90 days prior to study enrollment and must intend to initiate 2L systemic treatment - Participant must be willing and able to complete enrollment and follow-up health-related quality of life (HRQoL) and social support instruments - Participants volunteering for the Tissue Sub-Study must consent for use of their blood/tumor biopsies, which were collected as per standard of care, for exploratory analyses Exclusion Criteria: - Participant whose prior start and end date of DLBCL or FL treatment, and prior treatment received, including chemotherapy, radiation, surgery (not including excisional biopsies), and other anticancer therapy, are unknown - Participant who has any other active malignancy (non-DLBCL or non-FL) for which the participant is receiving treatment at the time of enrollment or any other former malignancy that was diagnosed within 6 months prior to Registry enrollment (with the exception of non-melanoma skin cancer) - Currently enrolled in any interventional clinical trial where the participant is being treated with an investigational product that cannot be identified