The topic of my thesis is the measurement and modeling of the thermal, electronic and optical behavior of monochrome Light Emitting Diodes (LEDs). The most important parameters of the laboratory measurements of LEDs are voltage, current, temperature and radiant flux. For my measurements I mainly used a thermal transient tester, an instrument for LED characterization with an integrating sphere and a spectrometer. I developed two procedures for transient measurement of radiant flux with these instruments. Later, I made a radiant flux detector circuit, which is faster than the spectrometer. In my thesis I presented the used instruments, the measurement setups and procedures. I reported my significant results of LEDs in steady state, including the diode characteristics and radiant flux, then I determined corresponding model parameters. After that, I analyzed the transient behavior of the LEDs in the electric, thermal and optical domain. I compared the results of the transients observed with different methods. Finally, I demonstrated a thermal model based on the transient measurements.