•Dual-function metal complexes possess both diagnostic and therapeutic properties in a single molecular entity.•Design strategies for transition metal complexes-based cancer diagnostics.•Structural design, mechanisms of action and strategies for different theranostic modalities. Cancer is an aggressive disease involving abnormal cell growth with the potential to invade other parts of the body. Despite intense interest from clinicians and biomedical researchers, the lack of complete understanding of the mechanisms of both the development and recurrence of cancer remains one of the greatest challenges in cancer therapy. With the continuous advancement of modern healthcare, the theranostic approach, which is regarded as the combination of therapy and diagnosis, has been viewed as a possible tool aiding the transformation of conventional, non-specific medicine towards contemporary precision or personalized medicine. Compared with traditional diagnostic modalities such as magnetic resonance imaging (MRI) and radionuclide imaging, optical imaging possesses a number of notable advantages that renders it as an attractive diagnostic modality. Among optical sensors, transition metal complexes have attracted recent attention as alternatives to conventional dyes due to their advantages of large Stokes shifts, long lifetimes, and ease of structural modifications. Representative examples of design strategies for transition metal complexes for cancer diagnostics and theranostics are discussed in this review, including chemo-theranostic, photo-triggered theranostic, targeted theranostic and combination theranostic strategies.