This chapter introduces the concept of carrier aggregation, the transmission of data over multiple carriers, with licensed and unlicensed bands in LTE and NR networks. It discusses carrier aggregation bandwidth classes, each supporting different aggregated bandwidth, each with specific characteristics and benefits. The chapter discusses the advantages of combining multiple Component Carriers to enhance data rates and overall network performance. Additionally, it highlights the evolution of carrier aggregation technology in LTE and NR, enabling the aggregation of both licensed and unlicensed bands for increased bandwidth and improved network efficiency.