Illicit drugs constitute a class of contaminants whose emissions to the environment has drawn attention due their potent pharmacological and biological activities. The monitoring of illicit drugs in wastewater and surface waters provides a powerful tool to investigate the presence, trends and to estimate the consumption of these psychoactive substances at the community level. The main aim of this thesis was to develop a solid-phase extraction (SPE) and ultra-high pressure liquid chromatography tandem mass spectrometry method for analysis (UHPLC-MS/MS) of illicit drugs in influent wastewater samples collected from four separate locations at different timepoints in Sweden and to assess differences in concentrations of illicit drugs among the four sites. The analysis of illicit drugs in wastewater is known to be challenging with regard to sample preparation, analyte recovery, ionization efficiency, as well as matrix effects. To overcome these challenges, an SPE UHPLC-MS/MS analytical method for the quantification of more than 30 illicit drugs was developed. Several parameters were optimized in order to improve the extraction, detection, and subsequent quantification of illicit drugs in wastewater samples. The results from the spike-tests show that SPE-UPLC-MS/MS method provided satisfactory recovery (69-133%), selectivity and linearity (R2= 0.997 ± 0.003). Three mobile phases were tested on the spike-tests whereof 5 mM ammonium formate gave the highest response for the majority of analytes and was thus selected. Finally, the method was applied to authentic influent wastewater samples collected at the municipal wastewater treatment plant from Umeå and Örebro as well as from wastewater samples collected from private wells from Örebro University and Örebro University Hospital. In total, 31 illicit drugs and their metabolites were targeted, among these 23 could be detected, out of which 12 could be accurately quantified and compared to other studies that monitored illicit drugs in major European cities. For the remaining 11 analytes, semi-quantitation was performed. Umeå had the most excessive use of amphetamine (386 ± 265 mg/1000p/day), cannabis (32 ± 16 mg/1000p/day) and benzodiazepines. Örebro on the other hand had higher abuse of cocaine (124 ± 60 mg/1000p/day), MDMA (25 ± 14 mg/1000p/day) and methamphetamine (2 ± 0.9 mg/1000p/day). Örebro University Hospital (USÖ) had the highest concentrations of opioids and ketamine, justified by their use as pain killers or anesthetics. Örebro University had detectable quantities of illicit drugs but in lower concentrations in comparison to the other sites. In conclusion, this thesis has successfully developed an analytical method for analysis of >30 illicit drugs in influent wastewater samples from four different sites in Sweden.