Marginal zone lymphomas (MZLs) are a heterogeneous group of indolent B-cell neoplasms, comprising extranodal, nodal, and splenic subtypes, each with distinct clinical behaviour and imaging characteristics. The role of [18F]FDG PET/CT in MZL remains a topic of ongoing discussion, as its variable [18]FDG avidity and site-specific sensitivity limit its general applicability. However, in selected clinical contexts, [18]FDG PET/CT provides valuable diagnostic, prognostic, and response-assessment information. This chapter reviews the current evidence supporting the use of [18]FDG PET/CT in MZL across the disease spectrum, including initial staging, restaging after therapy, relapse evaluation, and detection of histological transformation. [18]FDG PET/CT has proven particularly useful in non-gastric MALT lymphomas and nodal MZL, where metabolic activity is sufficient to allow response interpretation and prognostic stratification. Metrics such as complete metabolic response, ΔSUVmax, and SUVmax thresholds have demonstrated correlation with survival outcomes in multiple cohorts. In contrast, the role of PET/CT in gastric and splenic MZL remains more limited and largely reserved for cases with atypical features or suspected transformation. Emerging clinical applications include PET-guided biopsy, early transformation detection, and risk-adapted treatment strategies. The prognostic role of volumetric PET parameters (e.g. MTV, TLG) remains under investigation due to lack of methodological standardisation. While [18F]FDG PET/CT is not yet universally recommended in MZL, its integration into patient-specific algorithms, when guided by disease biology, site, and metabolic profile, may enhance precision in management.