Theoretical analysis of the evolution of viral populations in sequence space, and empirical fitness landscapes -- Self-organized maps to derive fitness landscapes from deep sequencing data -- Mutations to overcome viral resistance in plants -- The role of recombination in the generation of new begomovirus phenotypes -- Plant virus adaptation to new hosts -- Viral fitness, host interactions, and resistance to antiviral agents -- Viral fitness and evolution of hepatitis C virus -- Population dynamics and fitness variations of hemorrhagic RNA viruses -- Coronavirus genome sequences, variation, and disease implications -- Epilogue: viral populations from a theoretical and experimental perspective.