Because of the high fission yield, high mobility and long half-life, technetium-99 is considered to be one of the most important radionuclides in environmental trace application as well as nuclear waste management. The study on the determination of technetium and its speciation is therefore a key issue for understanding its fate and behaviour in ecosystem. This thesis aims to develop series of analytical methods for rapid and accurate determination of total 99Tc in environmental samples (e.g., seaweed, soil, and seawater), as well as speciation analysis of 99Tc in seaweeds. The application of 99Tc as an oceanographic tracer to investigate seawater movement and coastal pollution using seaweed as a bioindictor was also investigated. Analytical methods of 99Tc for different environmental samples such as seaweed and seawater were established. According to the types of sample, different pre-treatment methods including dry ashing and acid leaching for solid samples as well as co-precipitation of technetium using ferrous hydroxide from large volume water samples were investigated in details and a novel approach has been developed to separate and purify technetium from sample matrix and other interferences prior to measurement of 99Tc by inductively coupled plasma mass spectrometry (ICP-MS). Considering the special chemical property of technetium, the stability of technetium during dry ashing of solid samples and evaporation of sample solution was investigated and the corresponding pre-treatment procedures were proposed. To remove the interferences of ruthenium and molybdenum, extraction chromatographic techniques using TEVA column was applied. The sorption and elution behaviours of technetium, ruthenium and molybdenum onto TEVA resin were investigated and a possible mechanism controlling TcO4 - sorption at different concentrations of H+ was deduced. With the application of two small TEVA columns (1.5 mL for each), decontamination factors of >104 for molybdenum and >105 for ruthenium and recovery of 60-95% for technetium were achieved for different environmental samples. An absolute detection limit of 1.5 mBq for 99Tc was obtained by ICP-MS measurement. The analytical methods were proved to be reliable and have been successfully applied for the determination of 99Tc in environmental samples. An analytical method for chemical speciation of 99Tc in natural seaweed has been developed. Different species of technetium were separated using biochemical techniques and detected by ICP-MS after sample pre-treatment and purification of technetium using extraction chromatographic technology. Besides TcO4 -, most of technetium was found in bio-macromolecules of seaweed, such as algin, pigment, celluloses, etc. Possible binding modes of technetium with these bio-macromolecules were deduced. Seasonal variation of 99Tc concentration in seaweed (Fucus) was investigated by analysis of seaweed samples collected monthly in Danish coast, and a possible mechanism driving its seasonal variation was explored. The concentration factor of seaweed to technetium was found to be a constant and does not vary with sampling date. The results indicate that seaweed can be used as a reliable bio-indictor to monitor the level of 99Tc in seawater and estimate the temporal variation and dispersion of pollution in the marine system.