This chapter discusses theoretical issues that form an important background for this book. The distinction between linguistic and general phonetics is drawn. The social, linguistic, and historical centrality of the vernacular form of language is described. Some important theoretical issues in acoustic phonetics are discussed, such as the relations between acoustic categories -- like the useful concept of ``acoustic vowel'' -- and deeper categories; the definitions of allophony and coarticulation; some criteria for delimiting the set of acoustic phonetic features; and the basis for inferring audible differences from acoustic differences. Finally an experiment is described that repeats the fundamental work of Lisker (1949), with different results, which is attributed to the basic difference between phonetics and phonology.