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) - Primary mediastinal B-cell lymphoma (PMBCL) - 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 and must have initiated 2L systemic treatment ≤ 60 days prior to enrollment - For second R/R DLBCL cohort, participant must have confirmed second R/R disease during or after 2L systemic treatment and must have initiated 3L systemic treatment ≤ 60 days prior to enrollment - For first R/R FL cohort, participant must have confirmed R/R disease (grade 1 to 3B or transformed) and must have initiated 2L systemic treatment ≤ 60 days prior to enrollment - For first R/R PMBCL cohort, participant must have confirmed first R/R disease during or after 1L systemic treatment and must have initiated 2L systemic treatment ≤ 60 days prior to enrollment - 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, FL, or PMBCL 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, non-PMBCL, 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