Abstract Biocide chemicals are an essential control in the oil and gas industry. From drinking water to hydrocarbon production streams, it is necessary to use the correct chemical, at the correct dose to prevent uncontrolled microbiological activity. The objective of this review is to discuss an example of a biocide evaluation that tested seven (7) different biocide chemicals (from the same chemical vendor) against planktonic and sessile microbial populations by both traditional and molecular microbiological monitoring techniques. The methods used in the biocide evaluation were in accordance to internationally recognised standards; NACE TM0194-2014 ‘Field Monitoring of Bacterial Growth in Oil and Gas Systems’ and NACE TM0212-2012 ‘Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion on Internal Surfaces of Pipelines’. The procedure stated that 7 biocides at 2 concentrations (500ppm and 1000ppm of product) were to be tested against bacterial populations (planktonic and sessile), in relation to appropriate water chemistry and microbial consortia. The microbiological techniques used to determine the biocide efficacy were; traditional Most Probable Number (MPN) bacterial enumeration of; Sulphate Reducing Bacteria (SRB), General Heterotrophic Bacteria (GHB) and Acid-Producing General Heterotrophic Bacteria (APGHB), Quantitative Polymerase Chain Reaction (qPCR) with pre-selected primers for SRB, Sulphate Reducing Archaea (SRA), and lastly, Next Generation Sequencing (NGS). The results from the microbiological techniques allowed for an evaluation by ranking each biocide chemical in comparison to the total test group of chemicals, against untreated controls. Performance was based on the reduction of microbiological populations from the untreated control populations and greater microbiological community detail was achieved through interpretation of the molecular techniques data. The inclusion of molecular microbiological monitoring techniques to biocide evaluations is a novel approach to understanding the direct impact of biocide chemicals in greater detail. In turn, this approach will provide knowledge and valuable information for chemical addition optimisation and cost saving.