High-performance environmentally-benign (K,Na)NbO3-based (KNN) lead-free piezoceramics has been the spotlight in the search for functional oxides. The KNN-based piezoceramics with the multiscale structures have the superior piezoelectricity, thermal stability, fatigue resistance, mechanical properties, and reproducibility, for the practical applications. In this review, recent development on the multiscale structural engineering of piezoelectricity, thermal stability, fatigue resistance, mechanical properties, and reproducibility, and the emerging applications of KNN-based lead-free piezoceramics was summarized. In addition, the future perspective for the development of KNN-based piezoceramics was also presented.