In the CDM model, galaxies are formed in dark matter haloes, and, at early times, merge frequently with their neighbours. As time evolves (and redshift decreases), the rate of mergers decreases and the evolution of galaxies changes from being merger-driven to a more internally driven one. This change is progressive and the transition is very gradual. Generally, the internally driven evolution is on a much longer timescale than the merger-driven one. It is now usually termed secular (for slow), a term introduced by Kormendy (1979), who made in that paper the first steps in linking this evolution with galaxy morphology.
In the sixties, and partly through the seventies as well, theoretical work on galaxy dynamics was mainly analytical. The working hypothesis usually was that potentials are steady-state, or quasi-steady-state. Thus, given a potential or type of potential, theoretical work would follow the motions of individual particles, or would study collective effects aiming for self-consistent solutions, by following, e.g., the Boltzmann equation (Binney & Tremaine 2008). In this way, the basis of orbital structure theory was set and a considerable understanding of many dynamical effects was obtained. The advent of numerical simulations, however, made it clear that galaxies evolve with time, so that a quasi-steady-state approach can not give the complete picture.
Secular evolution was the general subject of this series of lectures, which were given in November 2011 in the XXIII Canary Islands Winter School of Astrophysics. My specific subject was bar-driven secular evolution and was presented from the theoretical viewpoint, although I included in many places comparisons with observations. In this written version I concentrate on a few specific topics, such as the angular momentum redistribution within the galaxy, the role of resonances in this redistribution, and its results on bar evolution and boxy/peanut bulges. I will discuss elsewhere the effects of gas and of halo triaxiality and clumpiness. The main tool I used was N-body simulations, and, albeit to a somewhat lesser extent, analytic work and orbital structure theory. It is only by coupling several independent approaches that the answer to complex questions, such as the ones we have tackled, can be obtained.
Introductory material, useful for a better appreciation of some aspects of bar evolution, can be found in Binney & Tremaine (2008), while further related material can be found in the reviews by Athanassoula (1984 - on spiral structure), Contopoulos & Grosbøl (1989 - on orbits), Sellwood & Wilkinson (1993 - on bars), Kormendy & Kennicutt (2004 - on secular evolution) and Athanassoula (2008a - on boxy/peanut and disky bulges), as well as in other chapters of this book.