This paper presents a formulation of lattice fermions applicable to all quark masses, large and small. We incorporate interactions from previous light-fermion and heavy-fermion methods, and thus ensure a smooth connection to these limiting cases. The couplings in improved actions are evaluated for arbitrary fermion mass~\(m_q\), without expansions around small- or large-mass limits. We treat both the action and external currents. By interpreting on-shell improvement criteria through the lattice theory's Hamiltonian, one finds that cutoff artifacts factorize into the form \(b_n(m_qa)[\vek{p}a]^{s_n}\), where \(\vek{p}\) is a momentum characteristic of the system under study, \(s_n\) is related to the dimension of the \(n\)th interaction, and \(b_n(m_qa)\) is a bounded function, numerically always~\(\order(1)\) or less. In heavy-quark systems \(\vek{p}\) is typically rather smaller than the fermion mass~\(m_q\). Therefore, artifacts of order \((m_qa)^s\) do not arise, even when \(m_qa\gsim1\). An important by-product of our analysis is an interpretation of the Wilson and Sheikholeslami-Wohlert actions applied to nonrelativistic fermions.