From February 7–11, 2011, the multidisciplinary Lorentz Workshop Circulating Tumor Cell (CTC) Isolation and Diagnostics: Toward Routine Clinical Use was held in Leiden (The Netherlands) to discuss progress and define challenges and potential solutions for development of clinically useful circulating tumor cell (CTC) diagnostics. CTCs, captured as "liquid biopsy" from blood, for counting and characterization using pathology and molecular assays, are expected to replace metastatic tissue biopsies to be used to predict drug response and resistance and to monitor therapy response and cancer recurrence. CTCs are highly heterogeneous; therefore, cancer type–specific isolation technologies, as well as complex clinical interpretation software, are required. Cancer Res; 71(18); 5955–60. ©2011 AACR.