  Measuring thermal properties of materials is not only of fundamental
importance in understanding the transport processes of energy carriers
(electrons and phonons) but also of practical interest in developing novel
materials with desired thermal conductivity for applications in energy,
electronics, and photonic systems. Over the past two decades, ultrafast
laser-based time-domain thermoreflectance (TDTR) has emerged and evolved as a
reliable, powerful, and versatile technique to measure the thermal properties
of a wide range of bulk and thin film materials and their interfaces. This
tutorial discusses the basics as well as the recent advances of the TDTR
technique and its applications in the thermal characterization of a variety of
materials. The tutorial begins with the fundamentals of the TDTR technique,
serving as a guideline for understanding the basic principles of this
technique. A diverse set of TDTR configurations that have been developed to
meet different measurement conditions are then presented, followed by several
variations of the TDTR technique that function similarly as the standard TDTR
but with their own unique features. This tutorial closes with a summary that
discusses the current limitations and proposes some directions for future
development.
