Purpose: To summarize the latest scientific literature regarding the concentrations of biomarkers in saliva and peri-implant crevicular fluid (PICF) of healthy implant (HI) patients and patients with peri-implant mucositis (PIM) and peri-implantitis (PI). Materials and Methods: The literature review was performed according to PRISMA guidelines. The databases used were PubMed MEDLINE, ScienceDirect, and Cochrane Library. A combination of keywords was used, and selection criteria were applied. Selected articles were published between February 1, 2017, and February 1, 2022, written in English, conducted in humans, and examined the levels of saliva and PICF biomarkers in PI patients and compared them to HI/PIM patients. Results: A total of 16 publications were selected, involving a total of 1,117 patients with 1,346 implants. Qualitative analysis revealed 49 different biomarkers, the levels of which were compared between groups. After evaluating the most frequently studied biomarkers, significantly higher values of IL-1β, RANKL, sRANKL, IL-6, TNF-α, TNFSF12, MMP2, and MMP8 levels were observed in the PI group than in the HI group. Conclusions: Of all 49 biomarkers evaluated, IL-1β and RANKL have potentially the highest diagnostic significance in the assessment of peri-implant inflammatory conditions, as differences were observed between all three groups (HI < PIM < PI), but data from current publications were not fully sufficient to provide strong evidence.