The living organisms are varied when viewed from a macroscopic perspective, but on the molecular level they function based on the same fundamental principles. All living organisms are compartmented into cells via cellular walls made of lipid membranes. Through the wall the cell needs to transport nutrients, waste, genetic information and signaling molecules. The cell achieves this task via passive and active transport. In this thesis we focus on passive transport processes. In Chapter 1 of this thesis I introduce the topic of passive transport and its importance for biological systems. In Chapter 2 I use fluorescence methods to determine the passive transport of molecules with self intrinsic fluorescence through lipid membranes. We have built a unique fluorescence microscope which is capable of visualizing the fluorescence of molecules excited with deep UV light. With this new tool we monitored passive transport through the lipid membrane for several biologically significant molecules like for example the bacterial signal indole. Indole is an organic compound linked to important cellular processes like bacterial growth rates and cellular morphology. It is believed that indole is actively transported through the membrane of Escherichia coli via influx and efflux pumps. Here we give an unambiguous proof that indole can freely diffuse through intact bacterial lipid membranes. We extend this study to other molecules with self intrinsic fluorescence , like for example the antibiotic norfloxacin and the fluorescent dye fluorescein. We show that both these molecules can undergo passive transport through the lipid membrane.