In Chapters 1 and 2 we present the 1.4 GHz and 345 MHz observations respectively. We discuss the observing and calibration strategy, the data reduction, and the final images obtained. Also discussed are the procedures followed for the source extraction at the two frequencies, the source catalogues (presented in Appendix A and B respectively), and the derived source counts. In Chapter 3 we present the LBA and HBA LOFAR observations and tests performed during the commissioning of the telescope. We describe the data flagging and calibration performed with the LOFAR Standard Imaging pipeline, and the image deconvolution. Chapter 4 presents the overall spectral index analysis, performed using the available multi-frequency information, including a 610 MHz catalogue from the literature, and a preliminary 150 MHz catalogue obtained from our best HBA commissioning observations. In order to use this catalogue we had to correct the 150 MHz flux densities for the primary beam attenuation as a function of distance from the image center. Such correction was obtained through a statistical procedure. The main results of the book are summarized in the final Conclusions.