Unraveling the role of proprotein convertase subtilisin kexin type 9 (PCSK9) as a key post-transcriptional regulator of the LDL receptor (LDLR) has shed light on its attractiveness as a target for LDL cholesterol (LDL-C) lowering. PCSK9 is a serine protease that is secreted by the liver upon post translational self-cleavage of its pro-domain. Once in circulation, the protein binds the hepatic LDLR, and upon internalization, the receptor is sequestered for lysosomal destruction. Gain-of-function mutations for PCSK9 induce familial hypercholesterolemia, whereas loss-of-function mutations display a broad range of mutation dependent LDL-C lowering. Initial mutation and knockout analyses established the correlation between PCSK9 and circulating LDL-C. Ensuing RT-PCR studies demonstrated statin induced up-regulation of PCSK9 mRNA expression. Subsequently, the development of novel monoclonal PCSK9 antibodies facilitated sensitive and robust ligand-binding assays that have produced concrete conclusions linking PCSK9 to LDL-C regulation. These methods have demonstrated the impacts of current lipid lowering therapies on levels of circulating human PCSK9; furthermore, they will be a key component in addressing the pharmacodynamic responses of first generation PCSK9 lowering therapeutics. Here, we summarize and assess the methodologies for PCSK9 measurement in the context of a review of events that have led to our present knowledge about this intriguing target. Moreover, we address conflicting data and discuss ongoing bioanalytical challenges in understanding the true circulating levels of PCSK9. Finally, we discuss the pharmacological impact of PCSK9 measurement as a screening tool and a biomarker for future LDL-C lowering therapeutics.