Compact, widely-tunable, continuous-wave (CW) and ultrashort-pulse laser sources in the visible spectral region are extremely valuable in a wide range of cutting-edge applications such as photomedicine, biophotonics and microscopy. The most promising approach to develop a compact, efficient and widely-tunable visible laser source is second harmonic generation (SHG) in a periodically poled nonlinear crystal containing a waveguide, which not only allows highly efficient frequency conversion even at low pump power levels but also offers an order-of-magnitude increase of wavelength range for efficient SHG by the utilization of multi-mode matching technique. In this respect, semiconductor lasers with their small size, high efficiency, reliability, low-cost and wide spectral range coverage are very promising for the realization of tunable visible laser sources. InAs/GaAs quantum-dot (QD) external-cavity diode lasers (ECDLs), owing to the unique features of QDs, are of special interest for that matter. The use of multi-mode matching technique and SHG in periodically-poled potassium titanyl phosphate (PPKTP) waveguides pumped by tunable QD-ECDLs led to the realization of compact, widely-tunable, visible, CW (567.7 - 629.1 nm and 574 – 647 nm) and picosecond-pulse (600 – 627 nm) laser sources. Furthermore, the ability of QD-ECDLs to generate two tunable longitudinal modes simultaneously allowed the demonstration of dual-wavelength SHG (505.4– 537.7nm wavelength region) from diode-pumped PPKTP waveguides. In addition, compact CW white-light and multicolor laser sources were demonstrated by the use of two ECDLs and a PPKTP waveguide. The demonstrated laser sources represent an important step towards the realization of a compact, room-temperature, tunable laser source in the visible spectral region.