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General background: As a result of human activity, soil resources have been contaminated with heavy metals and petroleum hydrocarbons. The great number of co-contaminated soils in the environment shows just how important it is to find remediation solutions adequate in such complex scenarios, which had seldom been studied before. Phytoremediation is a biological remediation technology, which takes advantage of the intrinsic physiological abilities of plants to remediate contaminated media. Plants and their associated microorganisms perform phytoremediation processes (e.g. phytoextraction and rhizodegradation), which can bring about the clean-up of cocontaminated soils. However, a major constraint which hinders the success of phytotechnologies is low bioavailability of pollutants in soil. As a result, chemically and biologically assisted phytoremediation are possible strategies used to overcome this limitation and enhance the efficiency of remediation. The chemical approach presented in this study involves adding biodegradable soil amendments to increase the ability of contaminants for being transferred from soil to plants and microorganisms. The biological strategy explored herein consists of inoculating contaminated soils with bacteria (bioaugmentation) able to improve remediation of pollutants and/or promote plant features.Main objectives: a) investigating the phytoremediation potential of alfalfa (Medicago sativa L.) in co-contaminated soils b) studying the effects of the low molecular weight organic acid citric acid and the surfactant Tween® 80 on the phytoremediation process c) assisting phytoremediation with a bioaugmentation approach using Pseudomonas aeruginosa bacteria.Methodologies: Determining germination and mortality rates, assessing plant physiological parameters. Quantifying plant biomass, heavy metals in plants, total petroleum hydrocarbons (TPH) in soil, soil microbiological indicators. Calculating phytoremediation parameters.Remarkable results: Alfalfa presented low tolerance to TPH contaminated soil at 8400 mg kg-1 soil, which was improved when TPH were present at a lower rate of concentration (3600 mg kg-1 soil). Alfalfa was able to take up limited quantities of metals (