Circulating tumor cells offer promise as a surrogate source of cancer cells that can be obtained in real time and may provide opportunities to evaluate predictive biomarkers that can guide treatment decisions. In this review, we consider some of the technical hurdles around CTC numbers and suitability of various CTC capture and analysis platforms for biomarker evaluation. In addition, we consider the potential regulatory hurdles to development of CTC-based diagnostics. Finally, we suggest a path for co-development of anticancer therapeutics with CTC-based diagnostics that could enable clinical validation and qualification of CTC-based assays as companion diagnostics.