The waves of practical interest in reflection seismology are usually complicated because the propagation velocities are generally complex. In this book, we have chosen to build up the complexity of the waves we consider, chapter by chapter. The simplest waves to understand are simple plane waves and spherical waves propagating through a constant-velocity medium. In seismology however, the earth's velocity is almost never well approximated by a constant. A good first approximation is to assume that the earth's velocity increases with depth. In this situation, the simple planar and circular wavefronts are modified by the effects of v(z). In this chapter we study the basic equations describing plane-like and spherical-like waves propagating in media where the velocity v(z) is a function only of depth. This is a reasonable starting point, even though it neglects the even more complicated distortions that occur when there are lateral velocity variations. We will also examine data that shows plane-like waves and spherical-like waves resulting when waves from a point source bounce back from a planar reflector.