The practice of "liquid biopsy" as a diagnostic, prognostic and theranostic tool 
in non-small cell lung cancer (NSCLC) patients is an appealing approach, at 
least in theory, since it is noninvasive and easily repeated. In particular, 
this approach allows patient monitoring during treatment, as well as the 
detection of different genomic alterations that are potentially accessible to 
targeted therapy or are associated with treatment resistance. However, clinical 
routine practice is slow to adopt the liquid biopsy. Several reasons may explain 
this: (I) the vast number of methods described for potential detection of 
circulating biomarkers, without a consensus on the ideal technical approach; 
(II) the multiplicity of potential biomarkers for evaluation, in particular, 
circulating tumor cells (CTCs) vs. circulating tumor DNA (ctDNA); (III) the 
difficulty in controlling the pre-analytical phase to obtain robust and 
reproducible results; (IV) the present cost of the currently available 
techniques, which limits accessibility to patients; (V) the turnaround time 
required to obtain results that are incompatible with the urgent need for 
delivery of treatment. The purpose of this review is to describe the main 
advances in the field of CTC and ctDNA detection in NSCLC patients and to 
compare the main advantages and disadvantages of these two approaches.